Under carriage downforce is not exactly an answer from heaven. If your suspension is not stiff enough you will get bad pitch reaction and under braking moves too much forces to the front of a car taking away rear grip. You will get into trouble when turning into a corner under braking. Reasons are, the car tilts forward and the front dives lower, making the front air dam even more effective because it blocks more air, the front under carriage becomes closer to the ground making for even better ground effect and to make things worse the rear lifts up making the rest of the under carriage a ventury to create even more vacuum on the front end while it takes away its potential at the rear. So for those who loves to put over done looking side skirts, beware.
It is this field that a wing is far better. The more you tilt the more the rear wing tilts and pushes the rear end minimalizing center of pressure movement. In general ground effect is sensitive to car behaviour. Take a sucker cup and pull it out, it won't move but when a slight opening happens it suddenly loses all its energy. That there also tells the strength of ground effect.
