That approach will not solve the issue that allied TD are very effective vs ALL vehicles.
Imo one should be more creative. One should test creating 2 set of rounds similar to Sherman 76 and use them to balance Super heavy tank and medium tanks separately. Unit meant to Counter Super heavies could have access to "AP rounds" with range, accuracy, penetration and ROF design vs those units , while normal round with characteristics better suited vs mediums tanks.
One could ever take a bit further increasing the target size of Super heavies (maybe decreasing of other vehicles also) so that thing become easier to balance. Finally one can create more interesting profile for vehicles and create a "flanker" clash that would benefit the most from engaging enemy units close.
Wouldn't it simply be easier to give allied TDs deflection damage and lower their pen? You get the best of both worlds the-armour makes units more durable without being trivialized but intended counters are always effective.
I'd like to see some tweaking with targer sizes as well but as a strictly minimum extra work maximum effect I think deflection damage really is the way to go forward.