The way I see it it's pretty simple. Anything I do to take myself off a straight line is going to cause extra drag. If I happen to be going downhill enough in a turn I MAY gain some energy, but otherwise it's waste. I think Newton had a law to that effect. I'm to tired to look it up. Good night.
Nice explanation on the airplane too. I've been in a turn tight enough to require full throttle to maintain altitude. It's pretty fun. Especially when transitioning from turning one way to the other as practice for steep turns. Passengers begin turning green.