I know you're asking about wings, but on a related subject, a well designed whole-body setup can and will improve your car's downforce WHILE at the same time lessening your drag co-efficient. A good example is the full-body kit from Mugen for the RSX. At over 4-thousand dollars for a kit, it is not cheap. There is a reason why. The kit is actually designed with wind-tunnel testing, which costs a LOT. Because of this, the kit...with wings front and back...actually increases the RSX's downforce, while at the same time reducing its drag co-efficient about .4...from .32 to .28.
What you need is to calculate how much top-speed you want sacrifice for better road grip...on a car with excellent road-holding already. If you are not quite a top-speed racer, it shouldn't really matter. Just know that downforce starts becoming a factor anytime after 40MPH, which CAN slow down your top-gear acceleration. Of course the faster you go, the more it becomes a factor.