bonj said:
if it's hardly noticeable it's not generating enough light.
But what at what percentage of effort does it stop becoming noticeable? 1%? 5%? 10%
Let's say someone is cycling along, not particularly hard, and generating 150W of power. Let's assume 95% of that gets to the bike - 142W. Now let's say that the dynamo uses up 5% of that power - 7.1W, and let's say the dynamo is 85% efficient - 6W.
A Cree LED can generate 130 Lumens/Watt (I think some can do 200 Lumens/Watt now) so you could get up to 780 Lumens from that 6 Watts.
780 Lumens is a lot of light, considerably more than most bike lighting systems. I'm not sure how many Lumens you feel you need for riding in the dark; I guess it depends how fast you want to go.
My guess is that I'd be quite happy riding in the proper dark with a decent 3W light, which would take 2.5% or less of the power I was putting in to the bike.