It makes plenty of downforce. Prototypes these days make a great chunk of downforce from the underbody. I watched it go through turn 1 (130mph+ at turn in) at Atlanta, the car was very fast. Now I am bummed I did not break out the radar detector to compare speeds.

EDIT: See the video posted by stevenrace above. It's a sort of controlled road test with some good engineering moments. In sum, there are a lot of subtle factors that make the car surprisingly stable and maneuverable - for more obvious reasons, it just doesn't behave exactly as we'd intuitively expect.