I'll chime in....my S3 has the dual-tipped ANSA muffler, IAP headers (love 'em) and what appears to be a replacement cat. I'm not sure about what sort of cat is on it, since the exhaust job was done before I got the car. At any rate, I get a great sound with the ANSA and IAP combo. Reminds me a little of the old Ferrari Testarossa sound from Need for Speed (old PC game.....I realize the two cars use vastly different engines, but if anyone happens to have ever played that game, well, there ya go). The idle rumble is nice, and the car produces a sweet voluminous little mini-roar when it winds out, if that makes sense.

Sounds pretty sweet also when you come off the gas at high RPMs.
I'm also told that the S3s had particularly poor-breathing stock manifolds, and that aftermarket headers help. No way for me to test this of course, but it's a good reason to go with headers (not sure how badly the S4s are affected by their stock manifolds).
Matt