The problem with this design is that a serious downdraft and consequent very low pressure area is created above the rear window and high presure air is moving along the side. So you have air moving in 2 directions and 2 differing pressures.
Can you say VORTEX? The drag created by spinning the the air dragging along 2 tornadoes is immense.
There is a slightly more efficient shape for creating drag, but not by much.
