not a bad idea bit the angles are a bit ineffective i think, at least it seems to me they could be even better.
you might want to make it look more like the volvo 3cc concept perhaps
Volvo's New 3CC :: ConceptCar automotive & design news
if the angle of the new roof is too steep the airflow won't stay attached. and the airflow on the sides is also very important
on the other hand this shape would promote airflow from the sides to merge with the airflow comming of the roof and while this would likely create two vortices at the sides it might also create a smaller wake than stock, and thus have overall less drag.