@Fresh The onewheel is only half the input to the equation, you're forgetting the rider.
All else equal a person with a narrow stance is going to be less balanced. Why would you give up a large portion of your balance in return for a small increase in OW efficiency? You do not ever know when an accident will occur, or how. However, when you do bail, the OW isn't going to save you. What will save you is your ability to operate with maximum leverage at the last moments of contact with the board/first moments of contact with the ground. Going with a narrow stance is giving up biomechanical efficiency, what saves you during a bail, for OW efficiency.