AW X5 with Black 215's is an amazing combination. Can't wait to see picts of it.
As for the looks of the AW X5 with 215 21" and sport suspension, you won't know until you see it. You could always lower it if you want a lower stance.
Also, most people don't like the BMW premium sound system to begin with, so I would say you aren't loosing much there. Maybe a speaker upgrade in the future would be what you want.