or should wattage not be confused with actual brightness of the device?
Yes, that's it. An illumination device has a conversion efficiency. Different types of devices convert the power used to produce light in a different manner.
So, don't confuse light output with energy consumption.
Amps is not the defining metric for power consumption. Watts is that definition. The equations posted are correct.
Light output is often measured in Lumens. Further, how the light is measured further complicates a lamps useful rating. A reflector can focus the light onto a smaller spot.
Then there is the spectrum of light to be measured. Is all the energy converted to visible spectrum? What spot size and at what distance?
If a bulb consumes 60 watts of power, it doesn't matter what the light output rating is (from a power availability perspective).
FYI, the stock CB750 headlight was a 50/40 Watt consumption device. This rating was made on the assumption it was provided with 12.8V. Because of it's resistive nature, higher voltages will cause an incandescent filament to draw more watts, and lower voltages will cause it the draw less watts. Note that the light output also changes this style bulb's light output brightness with voltage changes, as well.