At Bonneville one of the more popular stock bodied cars to race is the 50's Studebaker. Without mods to the body racers found above +/- 150mph the rearend became very light causing spins. The Studebaker racers pleaded with rules committee for some relief to the rules. Hence, the 50's Studebaker can be modified with two ducts running from underneath the body and exiting aft the rear window. The effect was to lower the pressure under the rear end and raise the pressure behind the rear window creating a higher downforce. Some classes do not allow a spoiler unless the spoiler is year specific OEM.
Regarding the "flush-kamm-back" Cobalt racecar: the kamm back or spoiler probably has two effects on the Cobalt one being downforce to increase traction/decrease wheel spin. Second the kamm effect is used to keep the center of pressure of the race car aft of the center of gravity.
Remember in the movie
World's Fastest Indian Burt Munro did a demo with a pencil, the pencil became stable when the CG was moved forward of the CP. Burt wanted to add weight to the front end of his MC to shift the CG forward, thus make it more stable.
The Cobalt kamm spoiler (creating drag) has the same effect, it moves the center of pressure aft of the CG. Many if not all Bonneville streamliners use the Kamm effect to stabilize the car at high speed.
IMHO a spoiler on a eco-car is for looks and does nothing to decrease the CD.