My Z3 experience was the exhaust did not add a whole lot to the engine's power. The sound difference was incredible.
Originally Posted by Evlengr
It sounds nice, but did it actually make a difference in response time?
The main power benefits I found were the Dinan software upgrades which increased redline cutoff and altered the fuel curve. This, coupled with the cold air intake for more air flow, gave quite a boost to my Z3.
As far as the 2007 and up X3's, I suspect BMW has extracted most of the power already. It is probably not worth it to invest too much into perfromance upgrades for the X3. I think you would spend a lot for a very little gain. The only exception would be supercharging. This could give you up to a 60 hp boost for the $5k range.
My garage: 2007 BMW Z4 3.0si Manual, 2007 BMW X3, 2006 BMW 530xi
Retired: 1998 BMW Z3
Member of BMWCCA!