Why do we keep going on about this already?
I agree that transitioning to 152mm can be an option for future tank design, time will tell. I agree that the Russian isn't 40 years behind in APFSDS performance, maybe 10 or 15 though, and that rebuilding older tanks with a bustle autoloader is no mean task.
I still consider that full-length rounds allow for heavier rounds with decent L/D ratio. AFAIK the British don't field anything with penetration ratios on the level of an M829E3 or DM-53. Now if you tell me they do, I'm ready to take the evidence into account.
I was just trying to point out that it would be on the whole easier, shorter and cheaper to develop new 125mm ammo for the current tanks and upgrades thereof, than developing a brand new 152mm-gun tank, as things stand these days
in the real world. Both options are not mutually exclusive and wouldn't serve exactly the same purpose, but I guess the Russian military would have to make a choice. That was just to support the "maybe not 152mm for real right now" argument.
Now as far as regards the Cold War 2020 mod, I have selected the 135mm in full agreement with myself, for various reasons ranging from compatibility with transitional tank designs to easier to model physics through ammo loadout.