1w will do fine as a helmet lamp to see the controls on the bars- repair the bike or read a map. Beyond that and you need something a bit more powerful.

I have a twin 5w Luxion and it throws out about 350 lumens. Just about good enough for 35 to 40 mph on the downhills but it is the spread of light that the "Quality" lamps give out. Pointless having a 5w lamp if it only extends 20 yards or give you a 100yard beam that is only 6ft wide. And I have had those lamps aswell.