From what I understand wings purpose is to create a downward force. The downward force is mostly needed during high speed for the wheels to grip the road. When car is going fast, its almost like floating/bouncing/skipping on top of the surface, the wing creates a downward push on the car to keep it on the road.

So for this reason, its works during fast straight runs, I dont think it helps much during cornering or turning. Just like an airplane wing, the bigger the surface area, the more downward force it can create, but too much area (too big) can also become a drag.
The angle position of the wing is also important, too much angle and it also becomes a drag and will slow you down.

Since we are talking scale models, the effects might not even be tangible, specially off road. I can see it being very imporant on drag racing though.