my brother has a milltek sport exhaust for his audi b5 rs4. he is also running 100 cell high flow cats, and the thing virtually spits flames (well, it doesnt, but it makes that really cool overrun and backfire noise).
at idle, its stock sounding, but at wot and when accelerating, it is noticeably louder. a different pitch as well. on the freeway, it can get annoying, but being a euro, it has better insulation than a jap import where droning can get hard to live with. otherwise turn up the stereo.
also, derek is my brothers mechanic as well. top class workshop for those on the northern beaches