i dunno what it sounds like but Gibsons daughter drives a that huge pink barbie hummer with LV interior and what not. BITCH ha cenos16 can attest to it! but supposedly gibson is a pretty good company. Sorry for the helpless post but you now know about his daughter.
I've heard a Gibson cat back on a 5 cylinder. It wasn't very loud but it sounded decent, it'd be perfect for someone that just wants to be heard a little better at WOT but not at idle. I'd imagine it'd be about the same on an I4.