The E53 X5 3.0i shares the same engine with the E46 330i.
Luckily there are a lot more E46 owners that are exploring modding these cars and a lot of their experience is applicable to us 3.0i owners.
Those guys are making Ebay turbo kits work, they are reprogramming the factory Siemens engine management up to FI applications.
Check out E46fanatics.com, they are doing amazing things to these engines there.
Here's a nice guide for bolt on mods, check the forum for the wilder ones.
330i M54 Tuning