We've thought about that, but a computional fluid dynamics model suggests that a further extension has only minimal gains. So Wunibald Kamm was right
It also shows that further improvements mainly come from the front and the transistion from windscreen to roof. That last one might be a tricky one, but the right shape of 'wind screen wiper shield' might help to shape the flow in such a way that it creates a more efficient flow over the car.