The temperature settings in the software relate to the motor's temperature, which the controller monitors using the internal thyristor that produces a variable resistance
(measured between the Black and White leads on the Hall sensor connector) dependant upon the temperature of the stator windings.
I expect that the stator windings insulation and the motor's hall sensors are able to withstand the maximum temperature of 150?C, as the motor should cut out completely when this temperature is reached.
I would expect the controller
(and the motor) to run much cooler with a 2:1 gear reduction, as the motor would then be outputting half the torque to provide the same speed and torque output at the propeller.
Reducing the torque should also reduce the current being drawn by the motor, which means a lot less strain on the controller and therefore less unwanted heat being generated.
A 2:1 gear reduction would also reduce the load on the battery, which should result in a noticeable increase in the motor run time and therefore range.
If you motor was only drawing half the Amps to maintain your current cruising speed, your boat should be able to go twice as far as it currently can.
A 2:1 toothed belt drive reduction is unlikely to make a huge difference to the noise output and the mechanical losses would be minimal compared to the increase in overall efficiency.
The VEC controllers should have internal thermal protection, as page 5 of the
controller guide lists an 11 blink fault code for "Controller over-heat protection" which would be triggered
"When controller operation temperature is higher than default value", but it doesn't say what the default value is.
Alan