According to a source at M GmbH, any change of exhaust backpressure/exhaust design can have a serious impact on performance/throttle response.
Don't have more on this, sorry.

Maybe you should just wait until some X5/X6M owners gather some experience with aftermarket exhaust systems before you get one.

I'm in the same boat: my X5M sounds horrible. The exhaust sound is loud indeed but it sounds more like a turbo charged 4-cyl than a V8.