Congrats on your new car!

Yeah, people don't really mod the X5 because the typical kind of people who buy them aren't after performance. Most of those folks bought M cars or 3/4 series (I know I have a 335d modded as my toy car). If we really come right to it, the x5 is a school bus. The people who buy the X5 are less interested in performance and more interested space, looking richer than their honda pilot cousins, and having that floating living room couch ride.

With that said, sure there are performance mods. The N55 engine has lots of tunes available, there aren't specific tunes for the X5, its a tune for the engine. Check out burger tuning for a piggy back (which you can remove come warranty time). You can probably get about 360 crank hp our of the engine with a tune. http://www.burgertuning.com/N55_Stage2_Jb4.html

I'd skip an exhaust as its not going to make any real performance difference, but it will change the noise levels if thats what your after. But why on a 2.5 ton luxury car?