Units should not be balanced against other unit of the same faction but what they are facing (else could end up with endless circle of power creep).
Diversity is should be addressed via role/utility/play style/map usefulness/... in sort with what its unit bring to the table.
T-34/76 is very cost efficient due to its timing.
T-34/85 is the one of the most cost efficient main battle tanks.
KV-1 has been buffed too much exactly because it was being balanced around the T-34/85 and that is a mistake.
I disagree entirely. Baseline performance should be balanced by stock units, doctrinal units should balanced off of the stock units that have established the balance as long as the balance has been established between the factions core units, which I believe that tanks, frankly are, then as long as doctrinal tanks are balanced relative to their stock counterpart all is well.
Even at that, look at Ostheer. Doctrinal they can get the okw p4. It costs 145 fuel iirc, which is 25 fuel more than the standard p4. It gets ~55 more armour.
The KV1 costs about double that more than the t34 and *gasp* gets nearly double the armour. It gets other things too (more health and currently a damage modifier) but considering that the cost difference between the t34 and the KV1 is roughly the difference between a P4 and a Panther. And I don't think we need to delve into all the things the Panther gets over the p4 eh?
The KV1s performance is balanced in the faction for its price.
The t34 and the p4 are balanced against each other, then slap an extra 50% to the cost of the t34 and its going to be a hell of a lot better...