how about a panel that fairs the bumper to the start of the trunk. it could serve as a partial grillblock, and maybe could have a slideing hatch it you can't fully block the grill.
an open grill will lower the Cd of a car by about 0.02 so that's a big gain for a simple mod.
remove the mudflaps and see if you might trim them down and install then in front of the wheels.
next i see the car has gutters around the windows... mabe some smoothing there could work wonders.
wheel farings in front of the wheels seems like a good idea, and perhaps lower the airdam a bit. and seen how smooth you can make the underbody.
the bub caps don't seem to bad so i wouldn't make that a prioriy, but smoother is always better (maybe you can steal the ones from you car you where parked next to
as for the rear fairing maybe you could try to make it like this
seems to me like the air going over the car has a much easier path than what's going round it so perhaps this shape will pay more attentin to that while the air from the roof will fall off the edge (like with a naca intake or with VG's....)
but what you show looks pretty prommising