You need to take you into account that the required power output scales quadratic with the speed. Power in itself means nothing, speed is what matters in the end. If you run the math (and various cycling blogs and magazines have done so), you end up with speed reductions in the order of less than 1% or fractions of a minute per hour cycled. You’d likely gain more by wearing clothing that has less drag.
8% less power would imply between 0.92 and towards 0.92^0.5 times lower speed depending on if you are in the mainly linear or mainly quadratic region of resistance.
Older dynamos with a bulb connected were quite tiresome to propell.
The 8% are at the top of the power output - at the bottom end, the fraction doesn't matter. You'll be able to provide 5W more if you're only pedaling at 50W.
The old dynamos were a chore. But their efficiency pales in comparison to a solid hub dynamo :)
