Is it true that (despite being a murderous dictator) Stalin transformed a fairly backwoods and largely illiterate peasant country into a world superpower in a matter of 30 years ?

A bit late, but here is my shot at it! This is a difficult question to answer because it basically requires us to know "What if Stalin hadn't existed/ruled/been born/been such a murderous bastard?", and we have no way of figuring that out with any real certainty of course. 'What if' questions are discouraged on the sub to prevent rampant speculation iirc, so in order to make sure we don't go full banana-wild-mass-guessing, I'll slightly morph your questions and split them up into three pieces:

What kind of policies were purposefully instituted to achieve industrialization by Stalin?
Did this transform the USSR from a backwards peasant country into a world superpower in 30 years? How does this compare to other countries?
Were the Stalinist policies a major reason for the growth?

And I'll take the defeat of Hitler as a side-order.

#The Industrialization of the USSR (circa 1920-1950) ^(and what wasn't normal about it)

The basic idea of the the collectivization of the farms, and simultaneously, the rapid industrialization of the USSR is fairly simple and sound. A primitive agrarian society has most of its labor stuck in unproductive farming. By taking as much surplus food and selling it on the international market for machine tools, and (forcefully) reassigning farmers to factories, the amount of industrial output can be artificially increased, which allows for subsequent modernization and productivity increases (read: tractors and fertilizer) for the farms, freeing up more labor and producing more surplus. Rinse and repeat.

The actual execution and the unintended side-effects of this plan were not simple and temporarily immensely destructive. The collectivization of the farms and elimination of the "kulaks" was based more on Communist dogmas than practical concerns about the productivity of the farm(er)s. While the various bad years in 1920-1950 might have caused famine in the USSR regardless of the collectivization, it is hardy disputed that the disruption from collectivization (and active resistance against it) had big negative consequences on the short-term food output, by the mass-death of cattle and horse population that was still necessary to work the land, the forceful relocation or reassignment of countless productive farmers (or their murder/starvation) to factories or new communal farms with lower efficiencies. More important than the countless of juicy anecdotes from this period, about faulty allocation of equipment and workers in the communal farms, is the net change in output and productivity. And much to the credit of the Five Year Plans, the manufacturing output of the USSR as a whole did increase by a whopping 127% in 1928-1932, and then another 112% in 1933–1937^1 ^2. The absolute numbers hide a staggering decrease in actual productivity however, as summarized in the aptly named paper "Was Stalin necessary for Russia's Economic Development?" :

TFP falls significantly in both sectors from 1928 to 1933: by 19 percent in agriculture from 1928 to 1932 and by 33 percent in non-agriculture.

There are many possible causes for this, but the most important one is generally considered to be the distortion of existing labor practices. The Western nations had industrialized over the past centuries largely through gradual market mechanics. A new invention was done, it slowly made its way into the factories/fields, making companies more profitable, and boosting output, leading to more capital, more discoveries, et cetera. Governments could and did encourage and discourage some practices, but the great industrializer the UK in particular greatly favored the laissez-fair approach of not distorting the market. The USSR wanted none of that, and by introducing peasants with little relevant skills into the factories and restructuring/destroying existing (agricultural) structures, the actual productivity per worker became significantly lower, which was only compensated on the national level by getting more workers in more useful occupations (read: factories). Other notable factors are that, while new workers were added to produce farming equipment such as tractors, the initial loss of manpower on the field and the aforementioned loss of livestock far outweighed the bonus to output from the new equipment on the short term. In the long term though, mechanization was the only logical choice from a production perspective. What is unique about the USSR's transformation is not just its planned economy however, it is also how Stalin dealt with problems, such as the famines, that arose. To boil it down: rather than having the entire country suffer a 'mild' famine, he closed the particularly poorly producing regions off entirely, kept requisitioning the same amounts of food and prevented the peasants from migrating; basically starved the poorer regions to death while they were still producing food for the rest of the country. This is something that probably wouldn't have happened under a less cold-blooded leader.

Another notable feat of the USSR was its literacy and higher education program. The USSR after WW1 suffered extremely high illiteracy, but in the lead-up to 1941 this was nearly completely eliminated at an unprecedented pace. In addition, seeing the need for engineers, technicians, physicists, doctors, et cetera, they greatly increased the higher education quotas, leading to spectacular numbers of students graduating every year. This is offset by some amount, by Stalin's paranoia, which tended to eliminate those with more power, position or knowledge. Many skilled clerks, managers and scientists were purged along with the officer corps and political enemies.

#Freaking Japan, man

There is little doubt that the USSR was a backwards, but large, peasant country after WW1. They were more akin to China than the Western nations. Skipping forward to 1950, it is little disputed that they -despite being bloodied severely- held the 'title' of superpower, a position shared only with the USA. Yet was this growth truly exceptional? It turns out that it is right in this period that a different nation showed up with an industrialization and economic growth that would put the USSR to shame. It's not the USA. It's Japan. Japan had a ludicrous expansion of its economy right after WW2. Despite being torn apart by war, having lost its colonies and spending several years under occupation, they would see a GDP growth of ~10% per year for over 2 decades. No matter what reasons we find, the Japanese non-stalinist, non-planned economy "economic miracle" serves as a counter-example to any claims of the superiority of the (USSR's) stalinist, planned economy. Still, it must be said that Japan had considerable advantages: the investments the US made in Japanese industries, as well as them being demilitarized, which actually increased the funds available for economic investment. Japan already very much had a "worker culture" and enjoyed a high level of education. This stands in great contrast with the early USSR, which of course forbade filthy capitalist from investing, whose early dealings on the world market were mostly constricted to exporting grain, and suffered from high illiteracy, a terrible lack of skilled workers, and the constant and immense need to keep a standing army of one half up to one million soldiers, along with considerable modernization programs for its large army, because of the consistent perception that the rest of the capitalist world was out to get them.