Great info! Before I installed the Bussman I was considering upgrading my headlights. After getting proper voltage to the headlights, the improvement was really noticeable and I didn't need to do the upgrade.I don't know the rules regarding outside links. You'd have to check with the administrator @Chris .
For incandescent and halogen bulbs, small changes in voltage do make a big difference.
First off, power = voltage^2 / resistance.
Hence going from 12 volts to 13.2 volts at the bulb (engine running) will result in a 21% increase in power.
Second, incandescent and halogen bulbs become more efficient the harder they are driven. I don't know the actual formula, but it has to do with the change in filament temperature changing the black body emission spectrum more into the visible range.
Manufacturers of incandescent and halogen bulbs have to balance efficiency vs life. This is why people complain about the brightest headlight bulbs burning out quickly or about how long life bulbs aren't bright enough.
Halogen bulbs are the same as an incandescent but are filled with inert gas and iodine/bromine gas. The halogen gas actually extends the life of the filament at higher temperatures (increasing efficiency and brightness) by re-depositing evaporated tungsten, so the manufacturers can run the bulbs far hotter (or longer life) than a typical incandescent.
LEDs typically have their own internal voltage regulator and are typically not affected by small changes in voltage.
I have that same clamp on meter.