Its not so much height that makes a spoiler work well, its angle of attack, shape, ect... As stated, I guess there is some height requirement or limit to make it track legal, I dont know which though. They probably dont want people not being able to see out of the rear because the wings in the way, so they make a height limit.
And let me note the top 3 finishers...
1 Beddor, Steve Yellow Ruf CTR 2 3:12.067 97.465
2 Rosenbaum, Amir Red Ferrari F40 3:17.922 94.582
3 Horzewski, Michael Blue Dodge Viper GTS 3:19.506 93.831
I will willingly admit it looks crazy and dumb, but it seems to work if he won.