As requested! I tried to get this done last night, but I don't think an iPod Touch has the ability to do a simple "Copy Paste"... and I didn't feel like typing in the link very bad...
As you can see, not a very good picture by any means, but it illustrates the point. The upper provides some downforce with the "lip" at the back end, and the lower one is more optimized for flow. I would guess the lower one could cut off around 15 degrees or so, and not have the lip. In practice, you could put the spoiler on the two sides of the car vertically to help a little more... that is if they don't interfere with the brake lights (make out of plexiglass?). Let me know what you think.