Noelle Motors (a BMW engine tuning company founded by Karl-Otto Noelle, who was responsible for the engine development at BMW ALPINA for 9 years) offers for the X6 xDrive50i performance upgrades from 468-560 hp with ecu, sport cats and turbo. For those in Europe it is quite interesting.
http://www.noellemotors.de/BMW_X6_E71.505.0.html