Looking at the testing of rear spoilers shows that they can reduce drag if properly used, such as the raised spoiler on Hot Rod Magazine's Bonneville-running Camaro.
From what I've read here, it's understood that having the spoiler will virtually decrease the angle of the rear end, tricking the airflow and resulting in smoother detaching.
This video shows Mercedes testing a dynamic rear spoiler:
I'm wondering if the spoiler would have an added effect. Vorticies are formed off a sedan's rear by air rushing to fill the low pressure area behind the rear window and over the trunk. If a spoiler was present, it would increase the air pressure on top of the trunk. Maybe this would help reduce the strength of the vorticies?
Thoughts?
Update: Visual aids
My idea is to turn this:
Into this:
Think of a Nascar-style spoiler, but not as large. Just enough to soften the virtual angle off the rear.