The GTI w12-650 retains the gross outline of the standard rear body but tucks in a wing good for 200mph and a bubbletop coupe body for wake reduction. No added length.
The Citroen Tubik may only have flow-through ventilation, but with appropriate inlets on the top and in the rear quarter windows could be topologically equivalent to the GTI top.
Somewhere in between? Open up the back and graft a hatchback body extension in place?
Edit: I almost forgot: