Not much! Some spirited souls have modified the air intake system. It's probably just a matter of time before more mods are available like what's around for the other Acura/Honda models.
Remember this is a cross vehicle, between a full fledged SUV, a wagon of sorts, and a luxury sedan.
It's not a street racer.
Given all that, we know anything on wheels gets modified eventually, whether it's approved by the manufacturer or not.
What does the sport pkg. of the X5 give you ? Outside of a sportier suspension option, you get mostly appearance options. Those are available, look at
www.acura.com or
www.hondacuraworld.com and of course you can find optional wheels/tires at several places.
Depends on what you are looking for, and what you consider a performance upgrade. IF you are talking about bolt-on superchagers, etc. just wait...