Agree with GTSBruce. I have the Corsa 3" cat back, and it sounds great to me (you asked about sound). I especially like the way it sounds at idle --- I walked around the car last night, and that deep uneven rumble at the tailpipes gives me goosebumps. The sounds generated makes me KNOW there is some serious HP under the hood.
Now I don't have to explain that it really could sound better, the factory has to meet noise restrictions, blah, blah, blah.