Hi Jb,
How much? Tough to put a number on it. But I say it's worth it in most cases. Most things run more efficient and last longer at cooler temperatures. I don't recall an airflow number but something along the lines of an automotive heater/ventilation blower or marine bilge ventilation fan has worked well for me and others I know. Compared to self ventilation (shaft mounted fan inside the motor), forced air cooling via duct from remote blower has several advantages. Those include cleaner and dryer air through the motor and more air through the motor. The air volume from the shaft mounted fan is dependent on motor speed. Forced air is not. So forced air is much superior at low speeds, especially beneficial going uphill slowly. And after a duration of high speed/high power travel, then encountering a stop and wait in traffic, without forced ventilation, the motor will experience heat soak. The motor benefits from having this heat removed quickly by the blower.
Downsides to forced air is the space of the blower and related hardware, as well as the energy used by the blower fan. If a constant running fan is undesirable, it can be activated with a thermal sensor in/on the motor. Depends on your installation and drive cycle.
Regards,
major