[skyking]

note that they are placed upstream of the (too steep) back glass, presumably to improve attachment there. It could show up in that case.
Where we used them was leading edges of airplanes, to preserve attached flow at a higher angle of attack. This was at little to no drag penalty. It was done after a great deal of research.
To simply stick some on like the back of that 'vette above, and make some claim is ludicrous.