Besides wheel well aero, could it have some front downforce effect?
Decreasing the gap might increase the pressure?
(I'm no engineer, just a shot in the dark?)
I've left mine stock and will be doing the brake ducts from the centre piece like Dubsesd had.
Oh and there is a thread like this already on the z4 forum:
http://www.z4-forum.com/forum/viewtopic.php?t=35339