self spotting for P4s is negligebly useful. As soon as it moves It wouldn't be able to shoot at what it spottted anyway. Im not saying some may find a use for it with p4 but it has no effect of making a p4 superior.
It lets you see that sneaky atgun behind your target. But yes scopes don't really have any synergy with PIV apart from the extra vision because it's a very mobile unit that already sees as far as it shoots.
Hull down is not devine enough to keep your p4 alive. Its a shoot me to death quickly sign.
It still can be left instantly, there's no real risk to it. That said not like I really use it either, it's great on paper but you almost never want to waste time on your infantry hulling down your tanks.
The pintle mg is not as good vs inf as the t34s main gun vs inf and the p4s main gun is mediocre vs inf so that does not give the p4 a anti inf advantage, and certainly not a far superior advantage .
PIV main gun is better than T34 main gun against infantry (and tanks too), PIV hull MG is better, PIV coaxial is better, and PIV has a pintle that has higher DPS than the T34 pintle+coaxial combined as well. Stop it with the "T34 is good against infantry" thing already, you can already see against OKW that T34s are pretty damn bad at killing any 5-6 man squads without going in for the crush.
As for the actual topic, I don't think t34/85s properly gated (so either an extra upgrade added somewhere, or just requiring both T3 and T4) and based on the single call-in cost from advanced warfare in the tech tree would really break anything. Don't think it would really fix anything either though, because it would still make sense to just save all your fuel and manpower by not teching because all the other vehicles except Katyusha are lackluster outside of their initial rush value.