We've talked about the physical mechanisms behind the operation of diffusers before:
http://tentenths.com/forum/showthrea...light=locost47
there's stuff in there about increasing massflow and Bernoulli and interactions with rear wings.
Just for the record, rear wings *can* assist diffuser (and front splitter!) performance on saloon cars - i've done it myself on at least half a dozen different cars, including the current Team Dynamics Civic which has its rear iwng right up high on the rear hatch but still helps out.
It doesn't have to be a wing, either. The NASCAR-style flat plate spoilers work too. Pretty much anything that can lower the base pressure or raise the height of the effective rear stagnation point will help increase flow under a car.