Two words come to mind, at least on the operational and strategic scale: Not very.

First off, let me start by saying that one of the single best sources for Soviet military operations in Afghanistan is Lester W. Grau's charmingly titled, "The Bear Went Over the Mountain" freely available here in .pdf form. Grau wrote / edited a similar book titled, "The Other Side of the Mountain" (freely available in .pdf form here) about mujahideen operations, often regarding the exact same engagements.

To get a clear understanding of the tactical realities in 1980's Afghanistan, you can hardly do better than to read through those two sources, though bear in mind that Grau was a recently retired US Army officer editing Frunze Academy documents for US Army eyes, so in my opinion, there is a slight tint of institutional schadenfreude in the text.

Back to your question though: I am presuming that by "Russian GRU Spesntaz," you are referring to the use of highly-trained airborne and reconnaissance troops in Afghanistan rather than any one specific unit of Russian soldiers. "SPETSNAZ" is less a specific unit like "Delta Force" or "SEAL Team Six" and more of a generic category that Grau translates as "Troops of Special Designation." For the purposes of "The Bear Went Over the Mountain," he defines SPETSNAZ units as:

Soviet forces trained for long-range reconnaissance, commando, and special forces type combat.

For the most part, they comprised well-trained conventional troops who were alternatively mechanized or air-mobile as necessary for the task. Throughout the vignettes, SPETSNAZ units numbering as small as 16 men and as large as multiple companies are shown to be more competent than their motor-rifle peers, but grossly misused for front-line combat.

One such example of the misuse of SPETSNAZ is their first mention in the vignettes: the March '86 assault on Xadighar Canyon. Four 16-man SPETSNAZ units formed a blocking position by air-assaulting key terrain and ambushing or calling artillery and airstrikes on fleeing mujahideen. They played their important part in a multi-battalion sweep-and-clear operation that netted... 20 Afghan fighters, plus enough equipment and munitions to arm 30 or so more mujahideen. Grau dryly notes that "Three hours of airstrikes is not a good way to achieve surprise. Again, the Soviet force seems to have engaged the rear guard and the 'uninformed,'" a pattern he points out in many, many vignettes.

Where SPETSNAZ units really shined was in the pre-war coup-de-main. Again, to directly quote Grau:

Soviet advisers disabled equipment, blocked arms rooms and prevented a coordinated Afghan military response. Soviet airborne and SPETSNAZ forces seized the Salang tunnel, key airfields, and key government and communications sites in Kabul. Soviet SPETSNAZ soldiers killed President Amin.

These were complex tasks on a rigid timetable that were absolutely required to stop any organized resistance from the Afghans before it started, and the SPETSNAZ pulled off each one efficiently and effectively. They were the exact same tasks they had performed in Czechoslovakia, and the tasks NATO was worried they would perform if the Cold War ever went Hot.

When the fighting didn't end after that initial paroxysm, however, the SPETSNAZ were grossly misused for understandable reasons. 40th Army had a serious disease problem, with anywhere between a quarter and a third of any particular line unit's strength invalid at any given time due to any of half-a-dozen infectious diseases. As a result, the better airborne and reconnaissance forces tended to be placed front and center during the action as the most combat capable unit.

This meant the duties they'd been pulled from were commonly not performed. Route reconnaissance, the development of operational and tactical intelligence, independent operations, and many other specialized tasks were simply left on the wayside meaning that "the Soviets often failed to find the mujahideen unless the mujahideen wanted them to."

Instead of properly supporting 40th Army operations, SPETSNAZ units often substituted for 40th Army or Afghan Government units. For instance, one vignette talks about how the 31st SPETSNAZ Reconnaissance group (a fancy name for 16 dudes with guns) manned a security post on a mountaintop. The commanding officer describes his preparations in exquisite detail, probably to cover his ass for his later admission that "About 1830 hours on 18 July, we were eating dinner and observation was lax. [Probably my lookouts had also decided to eat without my authorization]." The ensuing attack caught them off guard, but was repelled in a fashion hardly different from how a 40th Army or Afghan Government unit would have performed.

Ultimately, the verdict on SPETSNAZ performance in Afghanistan appears to be mixed. SPETSNAZ units executed their tasks admirably prior to and during the invasion. As in Hungary and Czechoslovakia before it, the Afghan government never stood a chance. Afterwards, SPETSNAZ units performed more efficiently and effectively on the battlefield than their motor-rifle peers, but they were grossly misused for line-unit duties, and their tasks did little-to-nothing in advancing the Soviet cause in Afghanistan, mostly because Soviet leadership was geared towards The Next War on the North German Plain and in the Fulda Gap, not toward what we now call "COIN warfare" in central Asia.

TL;DR: SPETSNAZ (well-trained air-assault / reconnaissance soldiers) did better in 1980's Afghanistan than 40th Army line units... but that's not saying much.