[Piwoslaw]

Originally Posted by AeroModder

I also thought that if the kamm was too short (such as simply detaching airflow off of the roofline), then it would just make it seem like the rear end was larger and produce a larger wake, since the airflow does reattach to the trunk right now. I guess I wasn't thinking about the flow around and above the vehicle as well.

A short roof extension wouldn't reduce your wake as much as reduce Cd (Cx) and lift (Cz). As ShadeTreeMech mentioned, your rear window is steeper than it should be. As the rear angle becomes steeper (starting from 0°), Cd gets smaller, with a minimum between 12° and 14°, and then grows until about 28°-30°, when airflow detaches. Lift increases with steepness, also until about 30°. For nice charts, see this post.

So what you want to start with is a short roof extension that will direct the airflow in the right direction, instead of allowing it "stick" to the rear window, increasing drag and lift. With a longer Kammback you start to get the added benefit of wake area reduction.

A second option is to raise and extend your trunk line. I believe that Frank Lee was interested in doing this with his Tempo, and Moonmonkey played with it on his Echo. Here are two threads on the idea:
Reattaching wake and drag penalty
Decklid mock-up photos