Don't forget that if you extend the top edge of the shape
past the tailgate you can then add a spoiler without increasing wake area. That is then likely to reduce drag and lift.
Also, you don't necessarily want a beautifully curved surface from the top to the sides, because this will encourage flow around this corner and so likely increase the strength of trailing vortices. The Tesla Model S has low vortex formation off the rear pillars - note their shape.
