How come? Apart from the good old Bazooka / PTRS / Panzerschreck story this is more like a popular myth. Anti-Tank guns are all similar in Penetration (US one with ability). Between equivalent tanks there is not a huge difference either. Like, P4 is slightly higher than T34/76 and Sherman on medium and long range but slightly lower than T34/85 and somewhat lower than Easy Eight Sherman.
Tiger is slightly lower than IS-2, Panther is a bit higher than IS-2, IS-2 is slightly higher than Tiger.
Jackson is slightly lower than Jagdpanzer IV, Jagdpanzer IV is slightly lower than SU-85.
All in all I don't see those huge differences and if you ask me it is easier for Allies to penetrate the Panther from the front than for Axis to penetrate the IS-2. But you know what, this is fine to me, I just cannot follow the argument of the OP.
I am in favour of your side armor suggestion by the way, I think this would make the game make more sense. Panther had historically excellent armor in the front, but on the side it was quite vulnerable.
Units with penetration 200 or higher:
OST: Panther (260), Tiger (220), Elephant (440), Pak40 (210), Pak43 (380).
OKW: Jadgpanzer (200), Panther (260), Jadgtiger (550), King Tiger (240), Raketen (200) and of course Pak43.
SU: SU85 (200), KV2 (200), IS2 (250), ZiS (200), ISU (was 260, now I'm not sure what is the value of AP shells right now).
For now it seems balanced. Both sides have units with nice penetration and huge armor, although there are issues like Jadgpanzer - SU85. I would pay more for SU85 to make it almost as useful as Jadgpaner. Of course Jadgtiger and Elephant deny any non-turret armor but that's different subject.
Now USF: Jackons (200). The end.
Simple question. How USF should engage Axis units if the best USF unit has 200 penetration?And what's more, Jackson has to be used at distance where it has.... 160 penetration.
Someone would say that USF units are designed to flank. Except Jadgtiger, Ele and KT (what's speed of KT with blitz?) Axis tanks are FASTER than Jackson. 2 volks squads near Jadgtiger and Jackson is dead. On the other hand Axis can flank IS, ISU with Panther and give a garden about PTRS and Zooks.
It's stupid when someone design faction to flank or engage at range but at the same time give enemy units with higer range and units that deny flanking.