I vaguely remember a feat that allowed sneak attack die to be changed to a d8 instead of a d6. Does anybody know of this feat and in which sourcebook it appeared?
Yep, that's the one. I at first thought it was overpowered, until I realized it was about the same as +1/die. Definately about the power of a feat I'd say.
Funny thing about that feat, it actually gets worse if you combine it with the one that makes your rolls d8s, but it stays just as good with the one that adds +1 to damage/die, even though the other two feats in question have identical benefits on average.