The 2cm gun can penetrate Stuart, T-70 as well as the AEC. It could also penetrate the T-34 in the turret or sides. There was also a 222 variant with a 28mm that has penetrated an IS 2 in the front.
Also the T-70 had 15 mm side armour. For comparison, the German K98 SMK H bullet had 19 mm of penetration. That's small arms fire penetrating the uber T-70 light tank.
Zero reason not to buff the 222.
Frankly, this doesn't really matter.
Here's what wikipedia says:
at 100m, standard AP round penetration: 23mm
Panzergranate 40 (tungsten cored AP rounds): 40mm penetration
front armor of Stuart/T70/AEC: 13-51mm (assuming that the higher value is the front armor)/45mm side& front, 60mm turret/16-65mm armor (assuming, again, that the higher value is the front armor).
So the way that company of heroes works, is that everything is scaled down. Normal sight range is 35meters. In reality, you would have AT guns firing accurately at stationary targets ~2200-1500m depending on the caliber of the guns. The maps aren't even that big.
There is no side armor in coh2. Thus, moderate penetration results when you've got a luchs/t70/222 firing at comparable light vehicles because the game is both scaled down and it allows players to get, in the case of light vehicles, more risky... like chasing a t70 down with a 222. you might win, you might not, but that first PaK shot certainly helps even things out.
Okay, and the way coh2 works is that when a shell is fired, it has a lot of different attributes regarding damage. Most tank rounds have a penetration value that, when divided by the armor value, gives you the % chance to penetrate a vehicle. These same rounds are also effective against infantry.
In reality, you didn't have access to all-purpose rounds. It was just AP or HE. Maybe HEAT rounds had some mixed effects, I really don't know, but if you want to have the 222 be able to switch between PzG.40 ammo and HE shells, be my guest, but keep in mind that you're shooting at TANKS and you're taking fire from 37mm/75mm guns in your flimsy little car. Would you like AP rounds with depleted uranium as some dlc? That's be nice.
Besides all of that,
222 is extremely cheap. But it's good at nothing, just boosting dps in engagements if you can kite and shit.
P3 in side tech in T2 for ostheer would help alleviate this issue. would allow ostheer player to stay in the mid game or hold out for P4/stug into the early part of the late game.
Also, that 2.8cm thing you're talking about is this:
https://en.wikipedia.org/wiki/2.8_cm_sPzB_41
while that'd certainly be fun to use, irl you had to close within 500m and I'm sure you had to get within 100m to be able to penetrate the lower glacis plate of the IS-1 tank. That's pretty close especially when the IS-1 had a medium caliber gun probably effective somewhere between 1000 and 1500m, if not more against stationary AT positions when firing HE rounds.