I'd think your biggest gain would be to put some plastic behind the grill opening (I've already made templates for mine) You could also run a chin spolier this will keep the air from going under the car and bouncing all around. Not sure cutting holes in the rear valance will help much. I have always heard running w/ the vents open helps but I warn you, make sure they are clean and free of debries... leaves, pine needles, nut shells, dirt make a heck of mess at 100mph AMHIK.