Water boils at a lower temperature at elevation due to a lower air pressure. Can't comment on the resulting loss of fuel efficiency due to the O2 reduction though. But I suspect the fuel inefficiency due to O2 reduction at altitude is not too severe.
As far as cooking goes some things need an actual temperature to be reached. Cooking at a lower temperature for a longer time won't do it. This would probably apply more to baking and not so much to rehydrating freeze dried food.
