[aerohead]

It's been considered since Prandtl's blown/suctioned wing research of the 1920s.
Georgia Tech continues research on it after decades of work.
So far,more energy is expended in moving the necessary volumes of air,than is saved with the modification.Also,the equipment can breakdown,something a streamlined shape cannot suffer.
The Ferrari 599XX race car has an intermittantly-used,porous, suctioned diffuser,enabled by two-trunk-mounted electric blowers which suck air from under and blow it cyclically through the taillights to detach the wake.
Together,all the 599XX mods drop the car to Cd 0.285,compared to Cd 0.336 for the base 599GTB.
Cd 0.285 is a long way from VW's passive,Cd 0.14 'flow body' of 1981.