U.C.L.A. did some testing on a Ford Taurus back in '97 in which they did a complete body overhaul. One of the things that they did was to add a short boat-tail to the car.
Something that I found interesting is that they hollowed out some of the area around the back license plate. Supposedly this helped as the airflow separated off the back.
Would you consider doing something like this on the Insights tail ? It would be an easy modification.
The link now appears to be dead, but here it is :
http://www.team-fate.net/technical/1997TechReport.pdf