Originally Posted by ChrisSlicks

None in my opinion.

I bought one thinking it would improve airflow and cooling but all it did was get beat to s**t by the sway bar.

Right now I run no undertray at all, which probably hurts straight line speed a little due to additional air getting underneath. If I get a properly vented hood (with the vents in the right places) then I will reinstall an undertray, probably the plastic one, although the Zspeed is stiffer.