Welllll...if you do it right, he flashes a new ECU; your stock one is untouched. Theoretically, all you have to do is swap back to the stock ecu and you are good to go...."XWorks ECU calibration is intended for offroad competition use only!"
Bummer! So, is there a way to switch back and forth without BMW knowing if there's warranty issues? Or is this permanent, and if you have an issue, you're F'd?