Vortex generators are far more effective at reducing rear lift than drag. You may see a few points taken off your Cd, but it won't show any returns at the pump. You will notice your rear end will feel more planted in high-speed corners, though.
Many newer BMWs have a single fin on the rear, which I believe is an antenna:
These also act as vortex generators, and should help the airflow remain attached on the back glass. Tuft testing is a quick and easy way to see where the problem areas are, and what improvements your modifications will do. But if you do decide to put the set of 10 fins on, I recommend placing them along the edge of the rear window. For the angles, I'd put them at (in order from the center to the sides): 15, 10, 5, 5, and 0 degrees.
Since you have a more popular sports car, I highly recommend looking into the racing equipment catalogs and ordering a full undertray. Cleaning up the underside definitely gives you an improvement.