> Has anyone ever fitted full belly pans under their Vanagon for increased
> aerodynamic efficiency? 

Our '85 Westfalia has a factory installed gas heater installed under the floor in the middle of the van.  This assembly is covered with a large pan the reaches from one frame rail to the other and from the back of the gas tank to the front of the transmission.  It has always been there so I can not say if it improves air flow.  
George
'85 Westfalia