I am curious what happens to respiratory rate when exercising with supplemental oxygen. You are all correct that you cannot increase haemaglobin concentration past 100% and therefore supplemental oxygen cannot meaningfully increase blood oxygen carrying capacity, but perhaps oxygen could decrease work of breathing because each breath contains more oxygen. In that case, less energy is being used by respiratory muscles and there may be a performance benefit. Carbon dioxide clearance may be a limiting factor, I don’t know. I’m curious in theory only - I have no intention of trying it out myself and can see no real world benefit!