The sport package suspension does drop it a little bit but it's minimal. And xdrive suspension rides higher by default so honestly it's not a huge difference at all imo. I've seen pre-lci's run 18's and 19's on stock xi suspension and both look good imo. Some people will say that 19's on stock awd suspension looks horrible, but you can decide for yourself.
check this thread for pics. Page 2, near the bottom has pics of both 18" and 19" setups on pre-lci e90 xi's with stock suspension.
http://www.e90post.com/forums/showthread.php?t=665540