Yup--I've had fun autocrossing the car. Now I work weekends, though, and since its primary function is as a commuter I've decided to take it in a different (and more economical) direction.
I've been reading all the boattail threads I can find on here, and the idea is really growing on me (especially with those potential Cd values Aerohead posted!). I'm thinking of a 36"-38" extension past the rear bumper (~60% of template), moving the license plate and stock taillight housings to the end of the taper, and building the tail out of foam blocks and fiberglass (using 3-wheeler's
method). I have to be able to see out the rear to have a legal mirror situation, so I'm thinking of using a piece of Lexan doubling as a hatch for trunk access, and removing the trunklid since it would be superfluous. This would also allow me to use the trunk edges as mounting points, in addition to a support on the bumper and perhaps in the taillight holes.