That brushless motor was rated at 85% efficiency, which does sound low for that type of motor. OTOH, A Mag S28-150 is listed as 82% efficient, so relatively, the Kelley motor is not too bad - scooter motors with ferrite magnets will be even lower efficiency.

The other thing to remember is that the Kelly motor is sensored so it will have better start-up torque than any of the HK brushless motors. I'm not saying they are the best motor ever, just a third type to evaluate after the hobby brushless and scooter types.

The other problem with R/C brushless out-runner motors for this application is that they are very open for cooling. Good in a plane but not near grass clippings; you would need to enclose the motor to stop clippings from clogging the motor and possibly catching fire.