according to wikipedia your Cd is should be about 0.30 so that's not a bad start, especially with the small frontal area.
if you draw a line from the top of the roof to the edge of the spoiler you come pretty close to 11 degrees so that's pretty good...
if you fill that in with a solid form you get something like this; wich i think isn't much different from what's already there :
personally id focus on the underside of the car, partial of full undertray, have a good look at the cooling system and see if the wheels could use dams or deflectors... lots of these things are found of modern cars so they really work, since your car is a little older it doesn't have these things but it's basic shape is has to be really decent already, the underside might look a little messy however, so personally i'd focus on thosr areas before venturing into altering the body of the car.