Here's a user review of the install and performance of the piggyback tuner by Burger on the X5. I had been monitoring this thread when I still had my X5 before deciding to trade in for the X1.
http://www.bimmerfest.com/forums/sho...d.php?t=654757
I'm personally going to refrain from upping the boost on the N20, as I plan to keep this car LT and want to wait a couple of years to see how this motor holds up, given its already running ~15lbs of boost in stock form.