To unlock E8 you need to pay 155/165F (not to mention unlocking nades or racks).
It's still more than Med HQ + Schwerer HQ, while I consider Schwerer HQ superior to Major.
I see no reason why last tier of OKW should be cheaper or in line with last tier of USF since it provides damn powerful units + locking down sector.
120F for Schwerer seems reasonable for me.
Indeed, I got the same feeling, I can get out Panthers out really quickly if I want. (Got 3x +5% armour bullitins for Pz4) so I always go for that one first, except if there's some heavy armour on the field already. T4 to 120 fuel is reasonable and more fair.