Here is what BMW says:
"The BMW Performance exhaust system invigorates engine output by allowing engine by-products to exit the vehicle more efficiently. RPM performance is heightened and engine performance is increased by up to 5hp and 3 ft-lbs"
Key words are UP TO which suggests best case. Since BMW posts all it power numbers at the crank not the wheel the exhaust does not do squat for performance but it does sound good.
Anyone who believes BMW is in error with their numbers should post before & after dyno runs.