Your photo shows the car in /Alt configuration at least. You can close up the radiator vents to benefit if you can cool it otherwise. My understanding is that the air dam does two major things: 1) keeping air from the generally high drag of rough underbody and 2) reduces front lift (and balance etc to the rear-changes to the front in a wind tunnel will show rear lift changes as well). In either of these circumstances you want the dam as low as possible and I can say from experience that you want some rubber/stiff plastic or something on the lower couple of inches as there are crunchies, berms, pot holes and other stuff awaiting on turn outs, return roads, etc that will bend it otherwise. Needs to be flexible enough to take an impact and reform. I don't have my wind tunnel stuff right now but initial configuration with the dam maybe 2.5" off the ground showed improvement by adding an inch to it. I definitely would not shorten it. JMHO.
