I think you could get away with more lumens, if it was a bigger light. I think bicycle headlights are just about 1/4 or less the area of your typical car (or even motorcycle) headlight, so a car's headlight doesn't have all that light concentrated in one small area, and the apparent brightness isn't as great for any given amount of lumens.

I do wonder if they make an actual "headlight" style headlight for bicycles, styled after motorcycle headlights.

Another thing I've noticed, since I've started seeing LED headlights on cars, is that the light from these LED headlights seem "harsher" on my eyes than the light from incandescent headlights.