I could have sworn I posted on this.. O_o
If the Qs tend to overheat the rotors, don't cover the wheels. Repair bills will outwiegh the costs of MPG gains there.
Also, the SLR McLaren has very large vents on the hood and quarter panels, so entirely blocking the engine compartment is a bad idea- it'll lead to overheating and shorter engine life. Make sure that whatever air comes in can easilly get out.
I'd set up the belly pan to be as flat as possible, covering the lowest point (probably the exhaust). Make sure you account for the heat of the catylitic converter, pipes, mufflers, etc and use the proper materials.
On diffusers- They provide a surface for attached airflow to curve upwards, and both produce downforce and reduce drag.