To the extent that the vent is at an angle to the flow you might get some turbulence bleeding off the trailing edge. A cavity facing forward will have a pocket of recirculating air.
See BMW's air curtain:
http://ecomodder.com/forum/showthrea...tml#post239013
That's a ten page thread from 2011. And here is a genuine BMW part for the left side for $16.95.
Genuine BMW Air Duct 'Air Curtain' Left - 51118068901 - Turner Motorsport