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For a range of crank lengths far wider than the narrow 165-180 range usually offered/used, there is no significant difference in biomechanical efficiency if gain ratio is held constant. See here for the most recent mainstream discussion of the matter.

While I agree with your statement that the rider will get used to whatever crank length they have and there is no mechanical efficiency as long as gain ratio is constant I still believe there would be an optimum length for the rider based on how their body has been formed (muscles, height, leg length etc,.) and the different RPMs involved. Even if that optimum is just for comfort.

Again, probably so little difference that it is not an optimum worth trying to achieve with so many others factors to worry about.