i imagine the pressure created by the air dam wouldn't extend farther forward than something similar to where the smoke bends to go around these lights. making me think that setting an air dam set back at all, will quickly diminish its affect. going more than an inch or so back may pretty much neuter the affect of the pressure in front of the air dam.
in the event that you don't get enough flow, reopening the grill to try to get enough air might even result some air flowing out through the holes, rather than through the radiator. as it sits, it sounds like you have a good front pan up to the radiator to work with, and you could just run a normal grill block. i might not of made mine a bottom breather, but the factory ducting just behind my bumper was already fubar. the differences might come out in the wash, but a bottom breather limits you from a more aggressive air dam at the very front of the car.