Well, first I just want to say again, that in my first post I was a little harsh and I felt bad about it. The fun of cars is to customize them to how you want, so I said sorry and didn't mean to hate on OP's.
Although I still believe that I think OP thinks this spoiler is doing more than it actually is.
First, for any downforce to be effective at all, you have to at least be going 60mph. Definitely easy to go 60, and I can see how sometimes on a track OP would be getting a little bit of downforce from it.
Except I don't think that he's going to be going 60mph through corners, to have the spoiler creating downforce to correct the rear-end from sliding out. Like he claims. Also, for the rear wing to be most effective, you should have some sort of frontal downforce on the car. So, that all the tires are getting downforce and having that added traction on each tire.
Honestly that's just what I've read about it. I haven't experienced or dealt with the physics of it, so I could definitely be wrong here.