[RedDevil]

Air dams and belly pans both lower drag under the car but in an opposing way.

Air dams raise the pressure ahead of the car (enlarge the more or less stationary bubble of air).
That's good because it makes the air flow over the car rather than under.
But the air dam cannot block all the air from flowing under the car if you want a practical ground clearance.

Belly pans allow the air to keep flowing under the car so they would lower the pressure ahead of the car, diminishing the size of the bubble. It directly reduces drag under the car so it can't be bad.
But if the air would move less fast (rather, more with the car) then the drag would be even lower.

I'd think the combination of a moderate air dam and improved belly pan would work best.
That would cause the air flow to follow the original (designed) pattern more or less, but with even less drag under the car than with either measure alone.