Just imagine all the downforce you'd get if you ran a wing mount arm from the front bumper up over the roof! It'd be awesome!That's changing the distance between the rear axle and the wing, which does change the leverage of the wing's downforce on the rear tire contact patch.
I'll admit I don't know the details; my assumption is that the position of the rear wing (relative to the body, which also means rear axle) is dictated by the rules, and does not change if the wing uprights are vertical vs. cantilevered. So the distance from the rear wing to the rear axle does not change in this case.