I think you are on the right track, if that video showed attached flow over the back window I would do exactly what you are doing. the effort to make it taller and still have a window is not worth it.
So keep up the good work and see what gains you get with it the way it is.
For a comparison to tuft testing, how did powdery snow stack on the rear window? did it slide off or pile up? In the winter I like to see where the snow fills in on my tail, its a good sign of a low spot or detached flow. it can be like a self correcting shape in a wind tunnel
