Like I've said before, there is a whole lot of real estate to manage underneath a car compared to the topside. Wind is very evident underneath. Some cars chassis, mine included, look like the New York skyline upside down. In that respect, painting stuff with more slick/slippery gloss paint helps it to be just that, slippery. But it does not hurt at all with the exception of cooling ducts to have it be more streamline with lightweight panels to evict unwanted turbulence.
