If I recall, Sport Mode gives you battery consumption data, which might aid in finding an answer to your question. Barring that....
It would be interesting to know if it's more economical to yaw into a side wind (like the big jets do) (ATTI mode), or if it's more economical to allow GPS mode to pitch the bird into the side wind. I've often wondered this myself. Because of the inherent inefficiency of a quadcopter (compare the energy used by a fixed wing for the same flight), I'd bet dollars to doughnuts that the difference between the fastest flight vs. the most economical flight are marginal. I mean, let's face it. A quad copter at ANY speed has the drag coefficient of a brick. And in most cases - like a car or fixed wing - wind drag is a major factor in finding the tipping point between "economy" and "diminishing returns." If one plots speed vs. fuel consumption on a graph, the point of diminishing returns becomes apparent because the curve is exponential. But in the case of a flying brick, I'd bet dollars to doughnuts that the curve is a little more linear because even it's most economical speed is still terribly UNeconomical. Let's face it. Even just hovering is a terrible waste of energy. I can't think of a better way to waste energy than to take a mass, strap 4 rotors to it, and hover. It would be like idling your car @ 4,000 rpm....leaving only 2,000 MORE RPM for top speed.
I'm just spit balling here. It think it makes for interesting discussion.
Discuss.
D