While I aggree that rarity should be an issue for wether something is perked or not, I think like in aircraft the supperiorty is more important.
Non perk-Panzer IV H, T34-85, M4a3-75
Cheaper perk M4A376(W), JpzerIVl70, Brumbar,isu122
Expensive Perk Panther VG, IS2m,JadgPanther, Tiger, Pershing
Very expensive KingTiger,SuperPershing,Panther II,IsIII