Hrmm. Not exactly the conventional wisdom. Though one would expect diminishing returns to kick in for those who have longer hours, so perhaps a more apt comparison (if impossible to calculate), would be productivity over a 35-hour work week.
One thing that does bother me in all the income literature is how no allowance is made for the value of leisure time. Even if a French worker is equally productive as an American worker, if the former chooses to consume more leisure, they do make less money, but they're happier than if they worked the extra hours. Having such a choice is a desirable thing, and let's not rail against people's utility functions.