[aerohead]

Slats and slots are for 2D-flow wings at very large angles of attack,to prevent stall during short field takeoff or landings.
A fastback car is more a half-body with 3D-flow.Since the belly of the car is flat,there is no camber,no angle of attack,and no lift.If the roofline is proper there won't be any separation,and separation-induced lift.
Spirit of EcoModder is more like the half-body with it's mods and is a zero-lift vehicle.