As I remember - the Speed Demon's rubber skirt (which runs about halfway to the rear of the car) is designed with something like 3/8" ground clearance. It's rubber, note -- and that means it can flex and even wear without doing much damage. And if it's less than a half-inch from the surface of the salt -- that reduces the amount of air getting under the car to a pretty low amount....
I'd love it if we could run that close to the ground but some of this also depends on the type car and maybe the class. We run comp couple so can do anything up there we want. Not sure if all classes allow an air dam so not sure what the OP can or can't do rule wise.
Hooley's Stude...
...is relatively wide and flat going into the wind. Something that will flex when hitting the crunchies will also probably be deflected back under the car over 200 mph and at even lower speeds, so could loose some of its effectiveness due to the force of the air on the front of it.
Speed Demon....
http://speeddemon.us/....on the other hand is very pointed in the front and like you mentioned the skirts are mainly oriented going down the sides of the car so don't need to be as strong as the air is not hitting them head on.
Hooley likes to try and keep the Stude looking like one as much as possible so the front air dam is made from fiberglass to resemble a front bumper. In pictures we have seen when the car is at speed it appears to be bending back some so we are adding stiff metal straps behind it for next year.
So low and close and flexible is good but be sure it is up to the job,
Sum