I highly question the inspiration behind this kit - and firmly believe that the inspiration was not for less drag.... If it were - they would have installed it on a completely stock Prius... Not on one with a Folgers can sticking out the back.
I'm willing to bet there's no under tray associated with that diffuser - rendering it useless. Why put one there if no one will see it?
Why add the window deflectors? Why change to whisk-like wheels?