That espionage was a major factor in the soviet bomb program seems uncontroversial. Wikipedia:

https://en.wikipedia.org/wiki/Soviet_atomic_bomb_project

Richard Rhodes is probably the definitive source for this history in The Making Of The Atomic Bomb and Dark Sun. Both excellent reads.

I don't say this to discount the abilities of the Soviet scientists working on the problem; it's an incredible achievement even with the extra "stolen" knowledge. But keep in mind that the US pursued four totally separate technologies for uranium separation and built massively expensive plants for each because they didn't know which was going to work out. The Soviets benefitted by avoiding a lot of very expensive and time-consuming dead ends.

I thought Dark Sun said that the Soviet team knew how to build a better design that a direct copy of the US implosion device but were forced to make a copy by political pressure from Beria?

Of course, that's why Soviets did put the first man into the space, they did the Venus exploration or they had the first orbital station.

Must be due to GRU spying. /s

For stealing Western tech?

Look at glorious Soviet cars, for example.

Now look at glorious Soviet massframe computers.