You can run them on 24 volts if you like.It's all down to the load you put on the motor and whether that keeps the internal power loss and temperature within limit.
You'll see MFA list a higher nominal voltage for the same motor when it is supplied with a gear assembly.
If the immersed prop is so big that it causes the motor to slow down below half the motor no-load speed then a reduction gear is needed no matter what the supply voltage is.
http://www.mabuchi-motor.co.jp/en_US/technic/t_0201.html shows a prop exerting the load torque on the motor should not be so big or coarsely pitched that the point of diminishing returns is passed (to the right of the maximum power).
A 550 motor peaks out with a larger prop than a 545 but a 1/12 scale Arun prop ,for example, cannot be driven directly by either of those motors.