[lunarhighway]

the hatch is the wrong angle for the flow to staty attached. the spoiler virtually decreases this angle. cars like a porche 911 or the saab 900 also use a big poiler at the rear, that actually gives them a profile more like a common sedan. and than theres also the pinto crisis fighter that used a similar spoiler like the one pictured. cars like the prius or insight all have fairly high rear ends, this is because the ideal slope of the rear for the flow to remain attached is not very steep so with a normal lenght this will result in a high rear.

the reson why a lot of cars don't do this is because it usually results in a split rear window.

the position of the spoiler is another matter of course as it can work in two ways. a full horizontal spoiler would extend the car and change the virtual angle of the rear but angling the spoiler up would reduce the length needed to get the same virtual angle, it would also make more downforce but also more drag (still the net effect would be an overall drag reduction).

since you already have some interesting data and i assume this car is not intended to become the ultimate aero car, i'd figure out what configuration of spoiler reduces drag the most and go with that. if you add an undertray and perhaps even tire dams you can further reduce the drag, and enhance the performance, without making any real visible changes the the classic shape.