So based on performance there are some factors that need to be taken into account when comparing Zis Guns and Pak 40s.
For TL
R skip to the BALANCING PROBLEM section...
Both of these AT guns perform very consistently against light vehicles and medium armour, the Pak very occasionally bounces T-34s but they have slopped armour so this can be deemed reasonable. If the Zis can bounce Pz4s I have not yet seen it happen in my 263 hours played but this isn't an issue.
FOR REFERENCE:
Against heavy armour there are some serious problems.
The Pak 40 statistically had 132mm of penetration at a 90 degree angle at 500 metres, so had to be far closer to penetrate IS-2 frontal armour. From a balance point of view it's very reasonable that a basic AT gun isn't likely to penetrate the frontal armour of a heavy tank. The pak doesn't have any issues with the KV series and performs fairly. When not comparing the Pak to the Zis, there are no issues.
The Soviet Zis gun, on the other hand, barely bounces when firing upon a tigers frontal armour, that's 100mm at a 90 degree angle. The Zis gun could not touch a tigers frontal armour unless it managed to strike a critical point like a vision port, so by that logic a Zis gun should not penetrate the tiger from the front most of the time. Zis guns only had 82mm of perpendicular penetration even at 100m.
Don't even get me started on the panther, 140mm (effective) sloped armour, Zis guns still seem to bounce very infrequently.
Then to make things even worse there's the elefant, 200mm of armour (even though not sloped). Zis Guns are a serious threat to even these beasts from the front.
BALANCING PROBLEM:
Now real life aside, it should be obvious what the issue is here. Zis guns effectively out perform Pak 40s simply because they actually hurt heavy tanks which they fire at. The Pak is regarded as the best AT gun in the game and while it does have the stun, in every other respect the Zis is just better
Pak40:
4 man crew
pens all tanks consistently excluding IS-2 and ISU (which it is useless against when firing at frontal armour)
stun ability
Zis Gun:
6 man crew, very good survivability vs mortars and grenades
Only has issues with the king tiger, it's a king tiger after all.
Based on the fact that Zis Guns can shut down panthers tigers and even elefants (although not so much elefants because you can sit outside Zis Gun range) while IS-2s will sit there bouncing consecutive frontal hits, the Zis gun is actually the better of the two AT guns.
The IS-2s armour could be considered the problem however it is a heavy tank and it;s fair enough that AT guns should have trouble, but this should be universal! The Zis gun does not deserve it's crazy effectiveness and superior usefulness over the Pak 40. The soviets have a tank destroyer option with an 85mm gun which they should be forced to rely on for dealing with heavier tanks as 2 AT guns should not be an immediate pain in the arse for german heavy armour, especially considering the soviets DO have other options which are redundant because the Zis gun's crazy goodness.
The Germans on the other hand, rely completely on the Pak 40 for long ranged AT support (unless elefant), while panzershreks are a decent supplement AT tool they still have issues with heavy armour and are not really worth the cost on pgrens, not to mention they really don't operate like tanks destroyers, more like glorified AT guns.
This issue really needs a fix, especially because of the poor neglected SU-85...