If the exhaust was left as is on the GFX rear diffuser that would have been cool. However, to get the most awesome affect, what if the exhaust came out the sides and boxed off the rear diffuser?
Something similar done with the keystone (beer) Camaro SS except the exhaust tips are before the rear tire which is a bad idea for a street car for anyone entering or exiting vehicle.
http://www.camaro5.com/forums/showthread.php?t=71515