Yeah, I buy that marginal productivity drops as you add more hours, but I would be shocked to find that sixteen hours is the point at which diminishing returns make additional work pointless. I guess for me the diminishing returns start kicking in closer to thirty hours of hard focused work, and I am by no means the best worker I know.
You are talking as if the employer's point of view is all that matters. Simply because you can produce a bit more useful work, that is, your marginal utility to the company is positive, doesn't mean you should. Marginal value is fine for a piece of machinery, or for the company to decide if they want you to work more, but it shouldn't be your reason to work more. Having fun, want the money, helping a friend; these are employee reasons.
But the original question was about whether these jobs exist. Given that there are significant coordination and communication costs as the number of people on a team increases, it's in the employer's best interest to get more (productive) hours from a smaller number of people. If someone working a 16-hr work week incurs more communication overhead than useful product, employers won't hire someone who only wants that arrangement.
