body kits usually weight much more then stock components... in addition, side skirts are add-on parts to something that is already aerodynamically working just fine.
and I'm sure that the designers of several of the top body kits spend several hundreds of hours in wind tunnels testing out wind drag on their $400 production body kits
bodkits are just for looks...
on an imports, its called "bodykit"
on a domestic, its called "ground EFFECTS"
I doubt that the creators of underbody NEON lighting made these things for any sort of improved driving functionality