Will a voltage step-up circuit increase the power output of a motor?
Unfortunately, there is no such thing as ideal. 

A motor has a maximum power that you can pump into it. The coil has a resistance and as such, you can only pump so much current through it or the thing will overheat and die.

A small DC motor rated at 12V is designed to withstand either being stalled, or running at some load torque, with the full 12V across the terminals indefinitely without overheating.

As such, doubling the voltage on a DC motor that is already running at full load will overheat the motor.

However, that does not mean you can not over-drive a motor. The trick is to limit the drive current to not exceed the maximum rated current.
Notice, that maximum current value also sets the maximum sustainable torque you can achieve with the motor.

IF the 12V motor, when it settles at a speed at 12V, is not drawing all of the rated current, it is possible to increase the voltage, and speed, till that maximum rated current value is being delivered. However that takes some fancy electronics to do. Obviously, if the 12V motor is already drawing the rated current at full load at 12V, you can no longer increase the voltage.

Notice though, you should not just apply that higher voltage from stopped. Doing so will significantly increase the startup current of the motor potentially damaging it.

The drawback with doing that is increasing the voltage also increases arcing on the commutator, this significantly adds heat and reduces the life of the motor. It also voids any warranty you may have had.

As for commercial vehicles, I can not comment. However, they may be boosting the voltage because the motors themselves are rated at double the battery voltage.