Can you put numbers on, or show a picture of, that curve? And what's the goal? Keeping the rear wheels planted or squeezing out more mileage?
Gurney flap (or wickerbill): small in scale, at right angles to to 'trip' the airflow locally.
'spoiler' spoiler: same scale as the rest of the car. I like to think of it as a fence that prevents the wake migrating up onto the decklid where it can suck on the back window.
Bonneville spoiler: flat and long. Benefits as you describe. It can also be the top of a box cavity:
