aero drag questions

ok, so after workign on my car for a while now and taking some top speed tests (not really to high since i have a type R tranny on 16 inch 40 series tires) so ok. back to the point.... has any one looked at the rear bumper, and seen them huge area's that just grab air.... behind the tires, all the way back...... ok. so here is my questions...

i have lowered my car quite a bit. I have no front lip..... now, to help with my air drag from the rear bumper, would a front lip ( extending it down 1-1.5inches) do anything? saying like anything at all? figuring i only have a few inched from the ground to the bumper... any one have any numbers or experience with it.... mpg, speed tests? cause after 135-140, it slows quite a bit....and you can hear the air just being punished under my car...