well i agree about spoilers, wings, and rear lips if dont't exposed to air and in order to canalize air flow, always depend of aerodynamic caracteristcs design of each car
i think this is used for canalize the floor flow and as you say decreases lift (remember that all down force hold back the car, but yes, cause it is not exposed, there is no drag), and about the turbulences what i was refering before may be has converge effects on top flow and decreases the rear lift, it's possible.....
but i still think that the 911's drop off is better for aerodynamics, well as you say decreases drag but not lift, well this is logical, not interesting for automotive issue but certainly aerodynamic(think about the best aerodynamic form: a drop of water); sure lips and wings help it, with their against, but in joint are looked for a good and measure job


it's an interesting and complex item that depends of a lot of factors


greets