If they started earlier, the USSR might be able to avoid the consequences of Gorbechev and his Perestoika and Glasnost. Or the USSR might appoint someone less radical then Gorbechev to General Secratary. But once we hit Gorbachev, the line is probably going to go down the same path as OTL.
The coup might have succeeded in 1991, but we are talking about the 1980s, not the 1990s.
Suppose that both countries recognized that their current systems were unsustainable in 1980, and they began the same economic reforms that China underwent later? Also, suppose they give the same limited freedoms China currently allows.
The Soviet Union -did- realize that its system was unsustainable, hence why Gorbechev was selected to take charge in the first place.
The Soviet Union did adopt economic reforms in reforming the state sector in the 1980s, the problem is that such reforms were ineffective because of the inherit power of the state sector to push back against any reform which threatens its power in a Communist society. Therefore you ended up keeping the fundamental inefficiencies of the state sector such as soft budget constraints (where state banks would just keep giving money to failing state enterprises) and economy of disscale (because SOEs tended to be mega-conglomerates). Not to mention state monopolies over the "commanding heights" which stifled innovation and competition.
China had the benefit of a country with a much smaller proportion of its economy in the industrial sector, and is hence capable of reforming without (at first) encroaching upon the urban working class and SOEs. By first adopting agricultural reforms (which were popular) and turning rural areas and townships into locations for building smaller, labour-intensive and semi-privatized industries which did not directly compete with most SOEs at the time stimulated economic growth and transformation away from the command economy. Since you can simply retain the old SOEs (again, at first) while creating new competitive industries which aren't part of the planned economy.
This is not possible in the USSR not only because the country was already industrialized with a proportion of urban population that even today is not reached by China but because the USSR was already an upper-Middle income country, which means China/Korea/Taiwan's model of labor-intensive export driven economy is entirely unsuitable for the USSR.
The Soviet Union can't. The actual Soviet industrial working class is too large and powerful to tolerate such changes within "actually-existing socialism" and it took years to get the youth of the nomenklatura to agree that ripping off the working class in one foul swoop was the way forward.
This is absolutely true