To use your light bulb analogy, it's like having a reflector or lense that focuses the light and makes it brighter in one spot. You may not need as much power to get the same amount of light in a concentrated area. And you may even be able to reduce the voltage input to the light bulb and get the same candle-power at a specific pinpoint location. But if you increase current through the filament past the limit of the bulb, you'll burn it out whether you have the reflector installed or not. Same thing with the speaker motor, whether installed in a horn or not.

Again, even with increased efficiency, we're still talking about a large amount of heat in the motors of a high-power horn. With excursion reduced because of horn loading, it is questionable whether the woofer's venting is as effective.