I don't have a strong feeling about this, but here's my inclination:
* For barbarians, Jeebs' point makes sense. All they're using is their own physical capacity. (Plus I think as a class barbarians are plenty strong as is.)
* For the magical enhancements, I sort of think the potion or spell wearing off should not kill/corpse you because it's magic. It wasn't your body pushing itself beyond its limits, but an external infusion of power. When it goes away, you lose the magic...not your regular old you.
Not going to fite anybody over this, though