Actually, all my posts on this thread were so far focused on a pessimistic scenario (i.e. a scenario in which the USSR is somehow magically unable to fix it's internal economic problems). However I don't really think that's fair, the Soviet people had a legion of bright economists in it's ranks and with a party and state leadership that is ready to listen to said economists, the Soviet Union could not only economically survive but in fact thrive.
To quote from Robert C. Allen's "Farm to Factory: A Renterpretation of the Soviet Industrial Revolution":
"The Soviet Union grew rapidly from 1928 to about 1970 by accumulating capital and creating industrial jobs for people otherwise inefficiently employed in agriculture. The strategy of building up heavy industry and the use of output targets and soft budgets were effective in doing this. The growth rate dropped abruptly after 1970 for external and internal reasons. The external reason was the Cold War, which diverted substantial R&D resources from civilian innovation to the military and cut the rate of productivity growth. The internal reason was the end of the surplus labor economy: unemployment in agriculture had been eliminated and the accessible natural resources of the country had been fully exploited. A new strategy was needed. The Soviet leaders responded to these changes by squandering vast sums on retooling old factorics and by throwing additional fortunes into Siberian development. It was as if the United States had decided to maintain the steel and auto industries of the Midwest by retooling the old plants and supplying them with ore and fuel from northern Canada instead of shutting down the Rust Belt and importing cars and steel from brand-new, state-of-the-art plants in Japan supplied with cheap raw materials from the Third World. What the country needed was a policy to close down old factories and shift their employees to new, high-productivity jobs, reductions in the use of energy and industrial materials, and increased involvement in world trade.The interpretation of the Soviet decline offered here is the reverse of the analyses that emphasize incentive problems and the resulting failure of managers to act in accord with the plans. On the contrary, the plans were implemented; the problem was that they did not make sense."
With the innevitable end of the arms race and with the party and state leadership solving the country's internal economic problems, productivity growth might well have gone up to 1.5% a year once again, which in turn could well have caused overall GDP growth to increase to 4% a year. And I haven't even mentioned the scientific-technical revolution and the introduction of computers and cybernetics into the system of economic planning yet. A Soviet analogue to Cybersyn (which is really only a matter of time. There
actually was a computer network that linked universities and factories across Leningrad in OTL) would've greatly streamlined the process of economic planning and would've minimized "management errors". As for the claim that such a system would innevitably suffer from GIGO problems, I don't think so. The state security and control commisions could gain insight into any transaction between factories and civil institutions with the click of a mouse, therefore any falsification could easily be tracked down. I think it is fair to say that such an improved system of economic planning could well have created an addtional 1% of GDP growth annually. So if the party and state leadership was able to fix the countries economic problems, the USSR could well (and realistically) have archieved GDP growth rates of 5% a year.
Now, let's do some math (yeay). Let's put this number into my model used before on this thread, while also reducing population growth somewhat (a reduction from 0.8% a year in the early 1980s to 0.6% a year in the 1990s and 2000s seems reasonable, considering rising living standarts).
In 1989 the Soviet Union's GDP amounted to 2.0 Trillion US Dollars. GDP per Capita amounted to 7.078 US Dollars.
In 2013 the countries of the former Soviet Union had a combined GDP of 2.75 Trillion US Dollars. GDP per Capita amounted to 9.503 US Dollars.
Now, considering the optimistic (but still very realistic) sceanrio outlined above, let's say that the annual average GDP growth of the USSR between 1989 and 2013 would've amounted to 5%. Let's assume that the annual average population growth rates would've amounted to 0.6%. The total population in 1989 amounted to 286.7 Million. If we stick with these estimates, the hypothetical GDP of the USSR in 2013 would have amounted to 4.4 Trillion US Dollars. The hypothetical population in 2013 would have amounted to 328.0 Million. The USSR's hypothetical GDP per Capita would have amounted to 13.414 US Dollars (which would imply an annual average GDP per Capita growth rate of 3.75% a year between 1989 and 2013. Such a rate of growth is not unrealistic, considering that East Germany had archieved an annual average GDP per Capita growth rate of 4.0% between 1950 and 1973).
To conclude:
In 2013 in OTL, the total population of the FSU amounted to 289.4 Million. GDP per Capita amounted to 8.700 US Dollars.
In 2013 (using this more optimistic but still realistic model), the population of the USSR would've amounted to 328.7 Million. GDP per Capita would have amounted to 13.414 US Dollars.
Living standarts would've been significantly higher than in OTL in such a scenario. In 2013, the USSR would've ranked 4th in terms of largest countries by GDP, behind the USA, China and Japan.