[Angel And The Wolf]

Do you not need tranny cooling?
Side skirts are used to keep air from spilling under the car from the sides, lessening ground effects. Not sure if keeping a partial vacuum under the car helps anything but cornering. Anything to help the airflow reconnect behind the car will help (boat tailing, diffusers under the rear etc)

Might try the old trick of a golf ball under the accelerator pedal, (if you can live with slower acceleration and passing.)