I am purely discussing it in the context of Relic's one heavy limit. The unit is fine as is, nothing about it should be changed performance wise, it's the matter of killing it while 2 other heavies are present on the field simultaneously.
Couple of problems:
1) You're thinking of the Sturmtiger from a team context, and ONLY from a team context- sure you play largely from team context but if you make such changes it affects other contexts;
2) Said team context is not always good (re dumbass teammates that don't support your flank or defense)- in such a case it won't matter how many STs KTs and JTs they can put out;
3) Not all games involve teams, unless you really think a suitable alternative is that when OKW player chooses Elite Armoured, he can never choose ST without sacrificing KT
4) This particularly context is about someone going Breakthrough Doctrine while the other goes Elite Armoured, these aren't the only two OKW doctrines to choose from, and sometimes people don't choose for synergy, they choose because they want so-and-so unit or ability
In any case I would think the entire point of this sort of synergy is to encourage- and reward- good teamplay. If you're suggesting that you can't have King Tiger if you deploy Sturmtiger, then no one will play that doctrine because ST is hardly comparable to KT in terms of role. At least Jagdtiger's role is much closer to KT's, and can just sit back and plonk at anything it can shoot at without much micromanagement, whereas Sturmtiger requires much more micro to keep alive and have an effect in the game. You'd have a point if ST can fire by itself without having to pull back for reload, at which time it is immobile and vulnerable to decrew....but that's not how ST works.
If there's anything to change it would be it's cost: at 560 MP, 160 FU, 18 popcap and unlocked a 11 CP.