Air dam or lift kit
 

I understand the purpose of the airdam, but why not lift a vehicle with a suspension modification to increase ground clearance? With a belly pan it might be easier to slide through the air than to move all the air over the top of a vehicle. Given enough room under a vehicle it would slide through the air more like a v shaped plow than pushing all the air over. With the increased ground clearance and the higher pressure under the vehicle the downforce would also be negated, putting less load on the tires. Granted more of the tire would show but that could be negligible if the bottom of the car could be cleaned up enough to allow smooth airflow.

You wouldn't have smooth airflow coming out the back, which would help kill FE. If you could find a way to smooth the airflow coming AND going - then you'd essentially be an airplane on 4 poles (tires).

You should calculate an estimation of how much more frontal area there'd be with that much tire exposed, then figure how much the Cd would have to improve simply to match what it was pre-lifted, let alone improve on it. IIRC belly pans have been generally credited with 10% aero improvement. Plus you'd want to fair the tires in some way so they don't muck up the flow around themselves and around that nice new pan.