For what its worth... I just went from a 3" split tapered to a 2.5" Borla catback exhaust (no cats) to a full 3" SZ with resonated test pipes, x-pipe and catback. Sound actually seems less loud. It's a noticably deeper tone, but I think its quieter overall. Subjectively, there is noticable power increase. Haven't dynoed it. My turbos are a little larger but my setup is otherwise pretty similar.