I don't really think that an unsupported heavy tank being countered by a tank destroyer is a problem. That is how it is supposed to be.
I'd agree if the price gap wasn't major. The firefly at 440MP 155F is just over half the price of the KT, but entirely nullifies it in any way if the KT doesn't have those raks there. A unit which costs 55% of another should not be shutting the more expensive unit down completely. You should be using relativley equal resources to engage your opponet to gain equal footing.
ISU, AVRE, elefant, croc... all these units I would say are slow and need support in varying levels, in the case of ISU even more. So why should KT be any different?
Pershing is the only heavy that can zap around and nuke everything, but most people consider it OP. IS-2 has KT armour with decent mobility but if you give it the maingun of the KT I would say it would be OP for sure.
If there's an issue with KT I would say some of vet is a bit mediocre, spearhead for instance could use a small buff.
I'd agree that those units need support in varying levels, but thats the key, varying. The croc can operate against most targets once these commander buffs go through. It'll have a full damage main gun and a flamer to clear AT guns. 2x ISU was used in the qualifier tourney last weekend by Rutra and dzzara against Hans and DevM. Hans and DevM didn't have the Ele and essentially couldn't counter them. I remember one of those ISUs walking into 3x raks and simply just drove away because of bounces and the single shot it landed vs the 3 AT guns. People used to complain about how the KT could walk into 2x AT guns and just clear them, but somehow that was different.
Point is, if a KT has desiginated support it can somewhat work, if it doesn't then it's practically unusable. With many other heavies they can operate alone. Not really gonna talk about the Pershing, it's hilariously OP but receives no nerfs.