According to a French study (Peugeot/Citroen), the large counter-rotating vortices I'm getting are
really reducing rear lift while also slightly reducing drag.
Their tests show that vortex generators are MUCH more effective at reducing lift than drag, and the large counter-rotating vortices I'm seeing on my rear window means that the VG's are doing their job quite well:
"It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant."
Here's the paper:
SpringerLink Home - Main
This is
exactly what I want- Reducing lift, with drag reduction as an added bonus.