[aerohead]

*The flat underside limits under body turbulence and in so doing,allows the flow field under the car to deliver useable kinetic energy to the wake area,and at a higher static pressure.
*Diffusers provide for non-separated flow 'up' the deceleration ramp,with simultaneous pressure regain,raising the base pressure behind the car,reducing pressure drag.
*The airflow volume is a constant,fixed by velocity.
*The golf ball effect is in effect,all over the car,above 20-mph.
*Dimpling the underside could act as turbulators although I wouldn't know the benefit.
Many modern underbody sheetmetal deformations have more to do with noise/vibration/harshness,altering a harmonic or resonant frequency of the belly so as not to telegraph certain frequencies into the passenger compartment.The sheetmetal is a sandwich construction.