The SU-100 stayed in service and then reserve for a long while, so it’s very likely they would have been equipped with post-war shells.
In the post-war period, Egypt used it the most, and they used BR-417B ammunition.
Also from what you have said, I think that gaijin should balance the tanks by choosing a period of the model, for example the SU-100 from 1944-1945, the same as the IS-2 (1944), giving them what they had at that time, since the IS-2 (1944) during WW2 did not have the APCBC or the smoke canisters.
They could do that, but then most tanks would probably come stock with their best round, which would go against incentivising spending GE for modules.
Be interesting to see it tested though, a lot of BR changes would be needed and for tanks like the T-72A, which gets a late production run upgrade, would that unlock a new shell?
It also happens with the BMP-1, that you can put the modification to convert it into the BMP-1D, which is at least 10 years more modern than the base model, maintaining the same Br, although it is also because of how they have left the ATGMs in the game. It would be interesting to separate them into two different tanks, or in any case to change their Br, but of course, with a change in the damage model before, so that the change in the ATGM apart from the guidance is reflected in the damage.
I wanted to make a thread about these sort of mods. Personally I’d like them to be different vehicles, as I like collecting.