The 20* comment was aimed at your proposed boat tail.
The flow over your rear window is probably not attached. Flow over the roof is, though. In your diagram you show the boat tail beginning at the crux of the roof and rear window, where flow would normally separate. Instead of letting it to that, you guide it onto the boat tail and continue the smooth flow to the end.
The Mitsu has VG's to put more pressure on its wing, creating downforce (as I understand it). Engineers likely spent days in the wind tunnel tuning this and its purpose is not drag reduction. Creating vortexes takes energy and that energy is coming from the car's engine. Forget about VG's!
