It's impressive that it works off the white wire at all... well more that the engine continues to fire that is heh, how about it's brightness at speed? The faster ya go - the better the power output of the magneto...
Problem is that a more powerful light would usually require more... well power lol and the mag just doesn't have that much to spare. I've one of Norm's lights: http://motorbicycling.com/f23/lights...e-no-7306.html
and while the voltage & wattage are about the same it is plenty bright enough... so I would assume that it's the reflector style that's making the difference.
You could try a better quality bulb however, even Walmart has a wide variety of 6v "lantern" bulbs and some of them are quite fancy with exotic filaments etc... You may wish to tighten up your plug gap if you're having problems with stalling and/or a "stumbling" idle BTW, I run the bike with (white wire) lights @ about .020 for that reason.