You could also try to make the ducktail spoler you allready have more effective.
In this autospeed article they talk about the vortex generators of the Mitsubishi Evo:
Blowing the Vortex part 2
(a quote: "The longer the arrow, the faster the airspeed."
..inside keversite joke)
They talk a lot about less drag in the article, but I think these vortex generators are also there to direct more air to the rear spoiler, making it more effective.
The new Beetle RSi also does this, but then with a rear window spoiler:
I think its primary function is to direct more airflow to the rear bonnet spoiler.
Just some food for thought.
Edit: I found the Mitsubishi research article, interesting read:
Research article