[Thalass]

You could try a slot around the back corners of the car (top and possibly sides) where the air is squeezed out at high speed, with the intake wherever, at the front grille, perhaps. Rather than a few big round ones.
They've tried that sort of thing on aircraft wings to keep the air energised and laminar at low speeds/high AoGs, but it wasn't really worth it as far as lift went.

Of course it may not be worth the extra energy and weight.