Hi,
I took the Perf. Control on my X1 but did not have the time to test it yet (still breaking in the engine).
I talked rapidly about it in this post:
http://e84.xbimmers.com/forums/showthread.php?t=1128327
I think what you're describing is due to the flex of your rear axle bushings.
I'll coment more when I have time.
Cheers.