Images From Flat Deck Lid
I'm not convinced that it's beneficial for lowering drag... Maybe reducing lift, I can see that, but I'm not so optimistic otherwise...
For reference... Here's the baseline
And one more baseline
EDIT:
I just measured my model and the Fusion 999
Rear Angle
My Model: ~27 degrees
Fusion 99: ~20 degrees
Profiles look similar, but that subtle 7 degrees is making a big difference...