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 :)
Older dynamos with a bulb connected were quite tiresome to propell.