Pan cars do much less slipping and sliding then TCs...if a pan car is sliding it is typically in trouble.
The FGX works better on rubber on our track because the independent rear suspension soaks up the bumps and the transition from banked curves to the infield and the ability to have rear toe in on the car makes it so you can get on the throttle much easier than with a pan car. On foam tires the pan car F1s will have a lot of grip so the bumps and punching out of a corner is negated...and the pan cars are lighter with less gear meshes so they will be quicker.