I bought several cats off Buildbase BMW at the beginning of last year, I knocked the catalytic filter out of a brand new one and fitted it to my bike. It looks totally original (cables, servo and stuff untouched) I also took the circular flap off the inside and left the spindle inside. It makes a much better bark than with the cat, everyone mentions the bang on the upshift. Never remapped it or anything and it does shoot flames out the back on occasions. Ive kept one intact for MOT purposes in the future.
Well worth decatting.
JimmyMac