There are a few extensive answers on here, but I think we can get to the crux of your question by comparing the two unification processes.
The fundamental difference between German Unification and Italian Unification is that unification in Germany was peaceful and unification in Italy was violent. German Unification was a mostly diplomatic process that started with an economic and customs union in 1818 (the Zollverein) and ended with the proclamation of Wilhelm of Prussia as Kaiser following his victory in the Franco-Prussian war in 1871. Although there certainly were strong pressures on the other German rulers by Prussian diplomats and politicians, the union was peaceful. The independent German rulers were able to preserve their roles in the new German Empire, which consequentially adopted a federal character.
On the other hand, Italy was unified through a series of wars in which the Kingdom of Sardinia-Piedmont forcibly removed the rulers of the other Italian states and held (possibly rigged) referendums asking the people if they would like to join the new Italian Kingdom. So post-unification history of Italy is characterized by a series of military occupations followed by a Piedmontese bureaucratic class dismantling the local administrative structures and replacing them with Piedmontese ones. Further, the consensus among Italian economic historians is that Southern Italy was an administrative mess, as the Kingdom of Naples could not or would not abandon its semi-feudal system of government. Centralized rule was seen as a means to construct a modern system of government.
Now, the Italian unification process was in great part fueled by endogenous activism; and activists were wont to take refuge in the Piedmontese Kingdom during the repressive post-Napoleonic restoration in the Italian states. These intellectuals would not only supply the Turinese government with a full roster of capable public administrators, but gradually entice it to take action bringing about the unification of Italy. So this meant that at the time of unification, although the incoming governing class was largely Piedmontese, there are significant contributors in and around the Piedmontese government from all parts of Italy. Milan was probably the largest contributor, but there was also an important contingent from the south, especially Naples. This might have contributed to the presence of prominent non-Piedmontese members of the post-unification political order which might mask or mitigate the fact that unification was achieved by Piedmontese conquest.
There's also a more nuanced discussion to be had with regards to the degree of dialogue between the Piedmontese administration and agitators in the rest of Italy. Indeed, it's important to examine the extent to which this dialogue shaped the unification process, which rather than a premeditated series of events is more accurately thought of as a succession of haphazard self-perpetuating actions. During the Kingdom of Sardinia-Piedmont's first intervention in in 1848 to support the Milanese revolt against the Austrian Empire, the precise sort of dynamic between Piedmont and Lombardy was up in the air; the Milanese had been largely in favor of a Napoleonic-style "Liberal State," and only in a second moment married the notions of liberal constitutionalism with Italian unity. Over the course of 1848, the currents alternatively petitioning the creation of an Italian Republic or a Lombard Republic had come to an uneasy compromise by being persuaded that King Charles Albert of Sardinia would transmute his capital to Milan to be crowned, like Napoleon, with the Iron Crown of Lombardy and found a new Piedmonto-Lombard Kingdom. If anything, there was consensus among Milanese leaders that they should engage with King Charles Albert as equals, and that Lombardy was not proposing an annexation to his existing Kingdom, but rather a union to create something new. But so tentative was the agreement that in spite of constant dialogue between Milan and Turin, when Charles Albert’s armies finally intervened against Austria they advanced very cautiously, unsure of the welcome the Lombards would save for them, and their slow occupation of the terretory would be one (but not the only) factor costing them the war.
The absorption of Northern Italy would go on to be conducted with the near-total consent of the local ruling class. The South, however, was obtained by conquest thanks to a semi-sanctioned military expedition organized by Garibaldi and the more radical activists. The nascent Italian government was in no way prepared for the sudden integration of a state that was already moribund and slowly imploding; resulting in the haphazard and altogether incomplete imposition of a Piedmontese administrative class onto the South, and indeed the majority of Italy.
AFAIK it was. It was the political force behind Italian indipendence and unification, thanks to a complicate web of alliances with the European powers (France and Prussia itself) woven by Camillo Benso, Count of Cavour, prime minister of the kingdom of Sardinia. With the help of first France and then Prussia, it managed to win against the much more prepared and numerous army of the Austro-Hungarian Empire. Other regions of Italy spontaneously voted to join the Kingdom of Sardinia - the House of Savoia was the first and only royal house of the Kingdom of Italy founded in 1861. There was a "lower" movement of the Italian people, embodied by Giuseppe Garibaldi, which put an end to Borbonic rule on Southern Italy through the famous "Spedizione dei Mille". He ended up giving these territories to Vittorio Emanuele II of Savoia, who intimated him to stop before trying to attack Rome (still under Papal rule), as he was worried that the popular insurrection would turn its head against the Kingdom of Sardinia itself. Garibaldi complied by writing a famous telegram in which he stated "Obbedisco" ("I obey"). Maybe someone can offer a more complex explanation, in the meantime I hope what I wrote helped clearing up some of your doubts.
I really appreciate all your answers but feel like some are missing the point of the question. I am asking if Sardinia Piedmont ever held a political position within the (unified) Italian Kingdom similar to that of Prussia in the German Empire, which is one of relative dominance.
Sardinia-Piedmont played a huge role in Italian unification, through its kings Charles Albert and Victor Emmanuel II and their armies.
Following the French revolution, liberal ideas in general, and that of a constitutional monarchy in particular, were gaining traction among liberals across Europe.
Charles Albert was part of a liberal revolution in Piedmont-Sardinia in 1821, and though he did not participate in the coup itself, he appointed regent in the aftermath and adopted a liberal constitution. Once the new king, Charles Felix, ascended to the throne however, this constitution was abolished again, and Charles Albert did not restore it when he succeeded Felix in 1831.
During the first decade or so of his reign, his policies were mostly pro-Austrian and intended to maintain his absolute (rather than constitutional) rule over Piedmont-Sardinia.
Because of his early support for liberal ideas, the Austrians saw him as a potential troublemaker. On the other end, the fact he later abandoned them in favor of absolute rule made the liberal agitators suspicious of his reliability as a leader of a unified, liberal Italy.
However, Charles Albert had been steadily building up his army, already the largest in Italy, during the late 1830's and early 1840's, and those looking to unify Italy knew that only with the aid of this army they could stand up to the Austrian empire.
Following a revolt in which the Island of Sicily ousted the Bourbon rulers, they adopted a constitution, hoisted the green, white and red flag, and praised the pope, Pius IX, who was widely seen as an advocate of moderate liberalization.
Prompted by this, the pope was forced by the population of Rome to adopt his own liberal reform package.
Charles Albert, who seemed to resent being curtailed by a constitution, but who had been courting those seeking to unify Italy, feared that he might lose his credibility and perhaps be deposed by a liberal revolt of his own, finally adopted a constitution as well. As part of these reforms, he also unified his different titles and holdings.
In 1848, after these liberal revolutions and reforms took place, discontentment with Austrian rule in their Italian dominions reach an all time high. Following revolts in Venice and Milan, the two largest cities in Austrian Italy, the Austrian army was forced to retreat into a number of fortresses in the area. Relations between the Austrians and the independent Italian states had also been poor for a while, in part because of the Austrian occupation of the Papal city of Ferrara after a rebellion in 1831, an occupation which they did not lift until 1859.
Seizing upon the revolts, Charles Albert of Sardinia-Piedmont declared war on the Austrian empire and marched to liberate and unite northern Italy. During this campaign, called the first Italian war of independence, he also adopted the Italian Tricolore as his flag.
The other Italian states all joined the war, mostly to appease their own population which were in favor of freeing fellow Italians from Austrian oppression, but most joined only reluctantly. This also split up the command of the Italian forces, which would plague their success during the campaign.
The Italian armies captured Milan, and meanwhile an uprising in Vienna, the Austrian capital, tied up the Austrian reinforcements. After some early progression by the Italian forces in pushing back the Austrians, they were finally held at the Adige river until Austrian reinforcements arrived, at which point the progress of the Piedmontese army was gradually rolled back. Demoralized and defeated, Charles signed an armistice, but tried attacking Austria again only 7 months later, and after losing this second campaign, was forced to abdicate in favor of his son Victor Emmanuel II.
Victor Emmanuel, however, shared the ambitions of his father. In 1858, Piedmont-Sardinia and France signed a secret treaty: France would aid Piedmont-Sardinia in a defensive war with Austria, and in return they would receive the Piedmontese counties of Savoy and Nice.
After securing French support, the Piedmontese government provoked the Austrians by performing a series of military operations close to the Austrian border, but within the territory of Piedmont-Sardinia. The Austrians demanded the demobilization of the Sardinian forces, and when this demand was not met, declared war on Piedmont-Sardinia in 1859, drawing France into the conflict. This became the second Italian war of independence. After a string of initial successes, this conflict ended when France signed a separate peace deal with Austria, receiving large parts of Lombardy including Milan and Mantua. These territories were transfered to Piedmont-Sardinia, which annexed them into its territory and ceeded Savoy and Nice to France in return.
In late 1859, the duchies of Parma, Tuscany and Modena merged to form the United Provinces of Central Italy, and in early 1860 it was decided in a referendum to merge the Kingdom of Sardinia and these provinces into the Kingdom of Italy under Victor Emmanuel II.
At this point, Italy was divided among 4 blocks of power:
In 1860, following the end of the second Italian war of independence, a Sardinian general by the name of Guiseppe Garibaldi took an army of 1200 volunteers, and conquered the island of Sicily and Calabria (the tip of the Italian boot) with the support of the local population. Following this success, the Kingdom of Italy invaded the Papal States and intended to push south until it met up with the army of Garibaldi. While king Victor Emmanuel fought and conquered papal lands, it should be noted that he did not make an attempt to conquer Rome itself at this point, instead moving west through Umbria. After a public referendum, the population of the occupied territories overwhelmingly voted to join the Kingdom of Italy.
In 1866, at the incistence of France, Italy signed an alliance with Prussia, Austria's main German rival. When War broke out between them, Italy joined the conflict, but performed very poorly. Because they had not accomplished much of anything in the Austro-Prussian conflict, they were not part of the peace talks, and Austria refused to ceede Venice directly to Italy. Instead, Prussia and France manage to convince Austria to hand over Venice to France, which in turn handed it to Italy, which held another referendum that resulted in the annexation of Venice.
Finally, in 1870 the Italian kingdom invaded and conquered Rome itself, completing the unification.
So, in short: Piedmont-Saridinia was the driving force behind Italian unification. It took advantage of the political mood at the time, and through a series of conflicts and diplomacy conquered all of Italy from foreign dynasties and absorbed other Italian states.