Even for the old style, if they are shaped like pockets with a solid backing, they will be forming their own circulating air cushion that's designed to push air outboard of the front wheels.
Note that most manufacturers nowadays have abandoned the circular foglight holes for square ones or holes with a tall vertical element in front of the wheel well, for just this effect.
You'd be better off doing a partial grill block... especially seeing as how the Mustang's grille and hood form a gigantic parachute...
