They want the noise.
There are really not any HP gains with an exhaust.
Look around here and other sites and listen to the clips. And compare to your budget.
I have a Borla on mine and like it. Better sound without being obnoxious. A bit of a drone around 2500 RPM, but I seldom drive at that RPM.
To get rid of the rasp, you need to replace the mid section. Personally, I like the sound.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)