I'm with RAArmstrong. Sell it and buy a diesel. Have you driven a diesel X5? At normal cruising speed, you can't really tell it's a diesel?
If you really don't like the sound of a diesel, I'd still sell the 3.0si and buy a V8 if you want better performance. If you're throwing money away trying to mod a 3.0si, you might as well pay for fuel on a V8 instead.