In theory... (bench racing here) if the rear window is cleaner then the air is staying attached to the car more than before. The point of the rear wind is, like a Kamm Tail, to "fool" the air into thinking the car is longer--the classic teardrop--, ergo the wing risers are having a negative effect on the aero of the car. .. It was designed, after all, by those who DID have access to a wind tunnel. .. Oh, air attached to the rear window will also make the rear end lighter at speed, as that attached air represent LIFT. ..
but then again I don't have a wind tunnel either..
my 2Ā¢