I can only speak for myself, but sound is a secondary objective. For me its about the performance, and lets face it, the catback isnt where the ponies lie. The HP gain and throttle response lies in the intake and the header - to a certain degree the cat.
IMHO the money - from a tuninggeeks POV - should be spent on the intake and the header. Sadly, the selection for the GDx is small yet, only Spoon and mugen so far. Im not too impressed with the gains from them either, but then again, its small engines, so what little there is to gain... right? Anyways - the important thing for any exhaust system on a NA'ed 4banger is resonance, explained
here, is the most important thing. Not only for performance, but also for sound. The better the resonation, the easier for the exhaust to exit the engine, and the smoother the sound. You know how cheap exhausts sound raspy/ricey? Thats beacuse they dont resonate the exhaust system for the car its made for :)