This doesn't actually seem like a great metric to measure by. If efficiency = constant use, then I just plain don't know the correct word to refer to a setup where all people are processed as fast as humanly possible.
Since the amount of time it takes a user to use a given toilet can not be increased (well, within the bounds of our conversation), efficiency becomes a property of number of available toilets and the level of use they experience. The only way to increase efficiency is to add more toilets, or to increase the rate of use of the toilets that exist (if there is waste).
With split bathrooms, the waste for one bathroom is likely high, since you can have a line out the door for one while the other remains unoccupied. This reduces efficiency.
So two unisex bathrooms would actually be startlingly inefficient, since the line that would ordinarily be in front of the lady's room is now divided between the two unisex bathrooms.
This statement makes absolutely no sense in that context. You are claiming that reducing the average wait time would somehow reduce efficiency - how, exactly? Maybe I just don't understand the argument that you made here.
Now, there is an argument to be made that requiring a unisex bathroom would reduce efficiency at times when the male and female bathrooms are both at max load, since urinals are more space efficient and thus more of them can be fit into the same area. If you adopt a urinal trough design, this gain in efficiency is greatly increased, obviously. But this is a special case situation, and could be better served by splitting into two bathrooms on a "urinals only bathroom" and a "stall only bathroom" to better meet demand.