You RARELY get dyno proven HP out of an exhaust mod on a BMW. The exception being the E46 M3, where they changed to cat's to meet US regulations.
You may get some power out of an intake, which is where you can usually get some power out of a BMW.

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2003 4.6is, Saphire Black on Black with Aluminum trim, Comfot Seats, Loading floor, Nav, TVonNav, Focal Utopia's in front, with Rainbow's in back, Helix 5 channel amp. ACS pedals, Aluminum steering wheel trim, X-pipe