Thats not quite the right question. Drop bars can be used long and low for aerodynamic efficiency. They can also be set much higher and closer for comfort.
In both positions you get a wide variety of alternate hand positions.
It is quite possible top set flat bars in an aerodynamic position, check out some courier setups.
The question of body positioning (separate from the shape of the bars) does have an impact on efficiency but it also depends on your flexability and how much power you like to apply to the pedals. Generally the more powerful the rider applies, the deepeer and more forward they prefer to ride. For pootling around town at low power a more upright position may be preferable.