I have a story (the pics are bad and don't show detail) of a Suzuki Mighty Boy that was overheating after an engine swap and a bodykit.
I'll have to find my notes but I think the pressure drop across the radiator at 80 km/h went from 0.15 to 0.25 inches of water. Went from overheating constantly at low speeds to thermofans only operating when stopped for over a minute (*really* small water capacity in a 550cc three-pot). It also dropped coolant temps at 80 by 5 degrees C
That was via sealing the lower intake to the radiator area and mostly blocking off the other side of the engine bay (it had one of those half-radiators, so I covered most of the gap). It was crudely done with Coroplast. I can find the photos and notes if you wish.
I have an instructable on prototyping undertrays
http://www.instructables.com/id/How-...-for-your-car/ that you are free to use, although your own work is superior.