No Reader's Digest version on this one, sorry. You kinda need to know why it's not simple, or you'll just ask "Why don't we do it the simple way?"

First step for you is to decide what flaws you notice the most, and what budget you'd dedicate towards addressing those flaws. Next is figuring out if you have a local shop that seems competent to respond to what you want changed.

Let's give the forum member who owns this 2005 X5 4.8 the chance to tell us about what he had us do... I may have already stolen too much of his thunder by talking about the OEM system ...