Yes I think the rear spoiler on the Pro Stock cars is only for aerodynamics. On Pro Stock I do not think any down force is needed. Most of their traction problems is in the first 60 feet unless they get out of the "Groove".
I would like to know if the little flat spoiler on your car helps or hurts the aero on the car. I know it would not be a huge difference but many factory style spoilers actually hurt mileage and top speed. I am considering putting a similar one on my HHR but will not do anything that may hurt it even if it is only just a little. I would not want to spend $250 dollars to lose 1/2 MPG.
I like the way it looks so if it is better even marginally I would put one on.
On an HHR you probably need a "whale tail" halfway down the back and a deflector up top.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
You could also do some tuft testing on your rear window to see if airflow remains attached all the way to the back, or if it separates before the spoiler. This would be easy to do since you could see the tufts in the rearview mirror while driving. Just use a few pieces of string 5 to 6" long, and duct tape them in various locations.