Yeah expensive ones still muffle, not usually as well as a stocker! A completely absent muffler would make an extremely loud and illegal car. I would be interested to what design goes into expensive mufflers, whether it be for best flow possible or mainly making a nice noise.
Interesting stuff though, 3kw is a difference! yes I agree any restriction loss whatsoever would have to be a positive one, but the tip is not the place to mod cars for power, 3kw is not much. If you were doing a full exhaust I guess having a good tip would be appropriate, every bit counts...