Also as the air is thinner...a/c have less drag so can fly faster and further when they're higher...doesn't work like that for heli's though..they rely on dense air to provide lift...
An other area I'd not really considered...but just have...higher up..where air is less dense and colder..that might make it more efficient as cold air when heated would expand more than half-warm air....I know what I mean in my own head...but can't get my words out..
