Cutting the springs or otherwise lowering the car is supposed to improve aerodynamics, although I haven't noticed appreciable gains since lowering with Eiback Pro-Kit springs.
I have been considering making vortex generators similar to the Lancer Evolution MR. I noticed on a rainy day that the rain just hangs out on the back window at highway speeds, indicating that the flow has detached and significant pressure drag is being created. I've noticed on some 80's BMW race cars that there is a spoiler at the trailing edge of the roof that diverts air down towards the rear window, presumably to reduce pressure drag. You might be able to develop some inexpensive homemade aero mods using some improvised flow visualization techniques to validate your designs. (i.e. streamers and a chase car or the dots of wet paint technique)
