I'm not sure if it was an option on the 2011 X5. 2010 X6 had a performance upgrade option that was a few hundred $ and bumped up HP by 40. They later rolled it out to the X5. It is now part of the M package on current models.
Price difference between new 50i and X5M is big, not sure what happens in the resale market. Good luck.
EDIT: I just went through this entire thread and couldn't find confirmation it was available in 2011. Plenty of questions whether it was available (reference to option code 7RR) but unless I missed something the question wasn't answered.
http://www.xoutpost.com/bmw-sav-foru...t-pics-12.html
Maybe LeMans knows....I tried but my attention span ran out.