Since I did not suggest a solution to the problem yet:
I suggest what I suggested in a previous discussion about the SU76: It does not need a straight stat buff. Give it utility instead. Make the barrage cheaper. Or give it something else, even if it's the StuGs stun shot (maybe limited to 50 range).
Muni reduction could work, it would give it something over ZiS at least.
Soviets used to be muni float faction, however over last 4 years it became one of the most muni heavy ones, literally just moving doesn't cost muni and even there are exceptions, allowing it to not be as demanding on barrage could be a way to revitalize the unit.
As you've said, stat buffs aren't really needed, its just about the costs.