It's an interesting idea, but very difficult to say for certain. it's very likey requirements such as headroom, crash safety regulations and the state of glass and metal construction at the time the car was designed had as much to do with the shape of windshield-roof transition than aerodynamics.
and while more modern cars seem to feature increasing smooth transitions
some pictures in this article suggest that the airflow stays attached pretty wel to even more aggressive angles.
autospeed
yet even if the new angle is superior the added frontal area might offset the gains...so some very carefull balanceing needs to be done.
on a more realistic note, if you had the resources to build such a structure, why waste them on the smoothest part of the car, when simply smoothing out the underside would improve the aerodynamics immensly.