Thanks for the answers! I have done some tuft-testing, but before I start on the kammback I should do it again (better).
It looks like there is some reattached flow on the rubber spoiler. The back window and middle of the trunk are all turbulence. I'm I seeing this right?
I can't (will not) drill holes in the roof to secure hinges, but I just realised I might be able to use the raingutter as a mounting point. I have a spare trunk that I can hack up so I might go for the full (to the bumper) kammback and make it hinge via the raingutters.