You can expect much better than 50% efficiency from this type of motor - more like 80-90%
At best it's the peak power rating, but how they define peak power is anyone's guess. Take it with a pinch of salt - I just go by the mass of the motor and dimensions of the stator. Bear in mind in their intended application these motors have a big propeller on the front blowing plenty of air through them which is clearly going to allow higher power than in our application which has barely any cooling by comparison. Ultimately the power rating is just a thermal limit, so if you can cool it well then you can get more power without the insulation breaking down prematurely. Either way it's plenty for what you need so I wouldn't worry too much...