I think your definition of productivity is off a bit. You need to account for "effective" productivity. My hours might be 40+ but how many of those are truly focused and productive?
Hence, working less time with (theoretically) better focus and reduced stress/strain would be more productive.
It's not my definition, but an econo-bs definition that involves aggregating $ value output, hours worked. So any study that discusses productivity will always find, at least over any short period, that hours are inverse to productivity as they cannot move in perfect lockstep. However, as an individual, your experience may (and probably should) vary quite a bit from what you might predict in the aggregate!
Hence, working less time with (theoretically) better focus and reduced stress/strain would be more productive.