15hp is not really a noticeable difference, not sure why you want to do that.
If you want more power, the best bet is a tune.
I highly recommend the N55 Burger tune I have in my car. It's +50hp and +80 torque for $500 bucks. The power is night and day.
2011 535i Alpine White, Oyster on black interior, Premium 1 & 2 Package, Magnaflow Full Cat-back Exhaust, JB4 Stage 2
Last edited by smashhell; 01-27-2013 at 05:17 PM.