"There may be some exceptional people who can defy this rule, and it's possible they are concentrated in startups, but I'm dubious."

Right. They are concentrated at the top. Here's one study (doesn't include work time at home or weekends): http://www.people.hbs.edu/rsadun/CEOs/presentation.pdf

It makes logical sense-- combine the ability to work long hours (rare) with talent and you will blow the normal fatigue-able people out of the water in terms of output and (in a sorta-meritocracy) rise to the top.