Aren't there inefficiencies in both ends of the process? If there's left over oxygen, that could be used in photosynthesis. Anyways, in a practical extraterrestrial farming situation, I'm sure inefficiencies in plant photosynthesis would be the least of the problems that could arise, and one of the most easily fixed. Just spray some oxygen at the plants and you're good to go. Also, inefficiencies are dependent on the limiting reactant. For example, say a photosynthesis reaction doesn't generate the ideal oxygen amount. if that plant is storing energy faster than it is using it, (growth?) there would be surplus oxygen that the plant doesn't need for oxidation. This would cover the inefficiency of photosynthesis. In any case, without actually trying to grow plants in an oxygenless atmosphere, this discussion is merely theoretical. Maybe this is something that's been tested, but I wouldn't know.