Adding an ally panel would definately improve the performance because otherwise the rear tub acts like a parachute!
My understanding (from a university project to design a track car) is that a diffuser has fins to create vortices and actually sucks the car down to road. If these aren't designed properly using a vastly overpriced flow analysis program you can end up with crazy results: acting like an aerofoil!
If you are going to put a panel under the back end (I'm going to) then i believe you need to have the filler pipe opening for the fuel tank external (not contained in the bodywork) to prevent gasses building up and, even worse, failing the IVA!
Hope this helps, no doubt someone will be along soon to give another, more experienced, viewpoint.