Yes, I have noticed the same thing. I believe I mentioned earlier in this thread that I have already tested the effect of opening the rear and side windows. I placed rows of yarn across the front, center and rear of the trunk lid. As soon at you start moving, the end row of yarn points to the rear while the yarn on the middle and front rows flip backwards toward the from of the car. I treid all combinations of opening windows and removing the roof and in none of the cases did it cause the front and mid row of yarn to flip toward the back. So I have concluded that the added drag from opening the windows will not help reduce drag at the rear.
Ryland suggested a spoiler at the roof angled down. Might have possibilities.
It's an interesting problem. Opening the windows seems to be a place where you would have a negative pressure because of the high airspeed on that part of the car. Of course opening the windows all the way defeats this but is quickly offset by drag. A spoiler is a tradeoff between lift (in this case downward deflection of air) and the induced drag that it creates.
Maybe if there was a way to duct air from a high pressure area (like the front base of the windshield) where there is already an air intake, it could be used to fill the void behind the rear window with less drag. Or more elaborately, take ram air from the front dam area of the car.
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein