the cats are a good size and not really that restrictive, besides a little back pressure helps bottom end power, not sure what aftermarket components will be useful. my 535 came with a flowmaster where the cat shoulda been and a glass pack where the muffler shoulda been, it sounded like **** and when i installed a stock headpipe and cat i got more power and smoother accelleration, i used a catback from a e32 750il with 2.5" pipes
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