The output from a dynohub (and I am including ones other than Sturmey Archer) will increase with speed. Don't assume a 3 watt or 2.4 watt or other watt rating and a 6 volt rating means very much. My SP PV-8 without any load on it will output about 34 volts AC when I ride down a hill at 25 mph, but once you start to supply a device (light, USB charger, both) with some current the voltage will drop.

I have come to the conclusion that the only time you notice differences in output from the hub are at low speeds where you are likely supplying too low a voltage for the device. I tried operating two LED headlights in series, I got double the light at normal riding speed but at slow speeds of about 5 or 6 mph, the lights were quite dim. A single light at those low speeds offered more light than the pair because at those low speeds the hub supplied adequate voltage and current for one light - but two lights cut the voltage in half and that was not adequate. At higher speeds the voltage climbed enough to supply both lights.

I also use an old bottle generator (actually an alternator) with incandescent bulb on my errand bike, that way I do not have to worry about bringing a battery powered light with me if I go shopping and come home after dark. Those lights are very voltage sensitive, I wired in some zener diodes to drain off excess power for going down hills so I do not loose my light at high speed. There was no over-voltage protection on those older lights, but that is built into the circuitry of modern LED headlights.