"Sags"? Try folds under at 150!
At normal highway speeds, it's not a problem, but if you are trying High-Speed runs, especially in desert heat, do not use the urethane.
Ours folded under at about 150. Sanctioning bodies do not like air dams that plow dirt off the course!
But they're great for parking lot stops, curbs, and porcupines that you hit on the road doing 45. Mine's hit all those things and other than a little burr from folding under after hitting the proky, you can't tell it's 10 years old! Miracle stuff for the street.