Old thread, but I have a car with a similar angle and I have post-VG tufts results. Unfortunately, I have no pre-VG installed results.
I have a high definition video on my hard drive that you can see the strings better. What it shows is that flow is generally attached throughout down the centerline, with a little bit of seperation right at the bottom of the rear slope, and reattaches throughout the trunk. Around the sides is a little bit more turbulent. The sides of the spoiler is not sealed to the trunk edge, and some airflow directs toward there.
The purpose of the tufts was to see if I can approximate a good height to mount an airfoil onto. I know this isn't a fuel economy mod, but I thought I would share.
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