http://www2.warwick.ac.uk/fac/sci/wm...tion120221.pdf
Once you get into simple aero mods, check this out. You might be able to throw a carefully trimmed piece of plexi onto the rear of the trunk lid so it sticks up and encapsulates some of the turbulence behind the rear window. Bring it up to the level of the Aero Template and see how it works! See page 6 in the PDF.
Since boat tails and kammbacks on sedans are notoriously difficult to pull off while maintaining trunk operation, this might be a middle ground good for one or two percent gain. Maybe you could figure out a flexible fabric or plastic kammback covering from there?