Honestly it should be allowed.
Humans have been cannibalist pretty much on every continent, with pretty much the same ideas of what powers you got from which parts.
Eating an opponents heart would give you, your opponents strength.
We've just evolved and made ethic and moral laws around that subject.
I find the idea awesome for both evil, neutral and good tribes. I don't see the reason for loss of/gain of faith in either ways. Perhaps a reputation boost if it is an enemy?