with cars like the CS, and Ford GT, a rear wing would affect the asethetic flow of the car greatly. further, it would not reduce drag, so other means of getting the applied forces were persued.
i made a shitty diagram to help...
one can reduce lift on a car by creating 'ground effect' which is induced by the front air splitter reducing the volume, and therefor, velocity of air under the chassis. this vacuume literally pulls the car down. to increase this effect, the underside is made flat.
the air that passes through the front also has some ducting to cool the front brakes, and an efficiant pass-through design to cool the radiator efficiantly causing less drag.
as the air passes towared the rear of the car, the venturis direct the air by causing them to vortex. this gets a larger mass of air to move predictably. as directed, the air fills in the vacuume behind the car, and literally forces the air up, and pulling the car down.
as air passes over the car, there is a high pressure zone on the windscreen, and allows less air to pass over the rear of the car, but rather deflected. the air that idoes pass over the roofline, is drawn down across the rear window, to the duck tail and then forced up before receding back to the vacuume.
ive only seen a carrera 4 997, so im unsure of the higher models; but the c4 doesnt appear to have any under side areo. the long tail design would produce less of a vacuume that the CS, or GT would, and underside areo wouldnt benefit the car much. im sure, however, that much thought went into designing the front air splitter for great ground effect to pull the front tires to the pavement.
with a retractable tail, or otherwise, drag is increased to stabalize the car at higher velocities by reducing the lift. without a ductailed spoiler on that lid, im unsure if any d/f is increased over the rear. im sure Stutgart doesnt want to have any flying porsches
if the underside of a 911 were flattened to increase ground effect, then perhaps the engine would be much more dificult to access, being that porsche boxer 6's really are quite cramped in the bootlid.
with the CGT, the rear spoiler raises at speed to create extra d/f by elevating itself from the vacuume behind the car, and reducing its tendency to revert back to it so quickly, im sure it would almost negate its effect.
im sure porshe has quite a few reasons why they havent used ground effect much yet in their production vehicles...