When discussing efficiency we are comparing performance between two or more alternatives. I would think that such a comparison would require variation for each person and type of effort. There would be a considerable difference between sprinting and climbing requiring much different type of pedaling. As an asthmatic, optimum for myself is that combination of cadence and power output that permits turning the pedals on my long rides, say 50 to 60 miles. Another way of saying this is if racing up the Empire State Building by the stairs, almost all of us would be better off taking the steps one at a time rather than two at a time.