Setting aside whether 60-80 hours per week is sustainable (and I think it's not for most pople, except perhaps for 20-somethings and people with hypomania), the research this article references says that there isn't a 1-to-1 correlation between hours and output. That's the whole point.

"... increasing a team’s hours in the office by 50 percent (from 40 to 60 hours) does not result in 50 percent more output (as Henry Ford could have told them). Most modern-day managers assume there will be a direct one-to-one correlation between extra hours and extra output, but they’re almost always wrong about this. In fact, the numbers may typically be something closer to 25-30 percent more work in 50 percent more time."

From about 1/3 down in the "The Overtime Exception" section.