[lunarhighway]

leaving it off will save weight so that might help, but if you're going to carry it that would be the best spot i think (unless you could put it on the front where it might serve as a grillblock and make the nose a little more pointy...) you could even argue that in a very crude way it extends the car and creates some sort of boattail.

in the real world i don't think it will result in any measurable gains. kammback lip spoilers at the roofline and the sides might give you better FE results and at the same time might cause less practical problems.