Originally Posted by surgeonstone

This.
Lets say you are going 20 mph with a dead on 20 mph tailwind. Up or down would be irrelevant. You would be cycling in a vacum of sorts with no penalty for sitting upright nor benefit from being in the drops.
But your feeling really powerful with this 20 mph tail wind and you crank it up to 28 mph. You now have an apparent headwind of 8mph and are going to feel resistance from the wind and a down position will help you.
But then you get all tuckered out and slow to 15 mph, still with the 20 mph dead on tail wind. Now you sit upright, arms out, erect, and gather the strength of the wind to help you along with no penalty.