Aren't spoilers of that type intended to keep the rear window clear of dust, rain, snow and the like?
If you want to see what the air flow is, buy a roll o surveyor's tape at Home Depot and tape 6 inch strips to the deck lid in a balanced pattern, side to side and front to back. Let the trailing edge point to the rear of the vehicle. Also tape them to your vent grills and maybe even a few on the rear window. Take it for a drive out I-20. Get the wifey to video the effect. Pan very slowly so you can view and analyze it. A lip spoiler on the very back end of the deck lid is supposed to help elimiate some of the backflow. I don't know how the fastback air flow is, but I did this on a notchback with spoiler and there was a lot of backflow.
