That said, productivity during extended 80 hour week crunches may well dip below 40 and be a net drag on the company, but it probably still wouldn't matter. Game studios in particular aren't asking themselves whether it's worth it to crunch, they don't know whether crunch is more or less productive and they don't care. They look at a looming deadline that appears to take 3x more work than there is time left. Publishers attach bonuses to being on time, and fees to being late. Crunch is often seen as the only option. That shouldn't be true at the huge game companies, but it still is.
Plus, it is really hard to measure true productivity in a non-factory setting like software. Nobody actually knows if 80 hours per week sustained equals 50 or 30 or 70 or 10.
All that said, a true 80 hour work week doesn't leave enough time & energy to eat properly, see your family, or even watch TV, let alone job hunt. I'm the kind of person that always does extra work, or has side projects going, but I realized at some point during game crunches that I was trading money for my entire life, and nobody can pay me enough to work 80 hours a week.
Not all jobs I had in the game industry had required crunch time, though. One had a development manager and veteran developer that didn't believe in crunch and never required it of us, and we never worked more than 50 hour weeks, and that was pretty rare.
I definitely recommend it if you can take the salary hit.
Being self-employed at the moment, I'm contracting to pay bills and working on my company, it's the same thing as a never ending crunch time! I certainly feel like I'm getting more done than a 40 hour week, but maybe I'm not getting enough rest to know the difference. :P
The way I solved it was by setting actual work hours and sticking to them. Anything that came in outside work hours could wait until the next day.
I feel like one of the worst parts about a compressed work schedule is that I've mostly stopped playing & exploring, and exploring is where the good stuff happens. Many of my most important personal developments and achievements were on "fun" time, even if it was work. Doing overtime and working too much like I'm doing now mutes that.
I have some commitments in the short term that mean I can't fix this very quickly, but long term I aim to be where you're at for sure.
The claim is not that 40 is some kind of hard threshold, and if you work any more than that you'll be instantly less productive. The claim is that a steady-state of, say, 60 hours per week is less productive long term. It's actually more productive for a little while, like a month, but then people get tired and less productive than rested 40-hour workers after that.
I have experience doing long periods of 80 hour weeks, when I was working in game development. I think it's hard to say definitively that productivity goes negative, that's not what it felt like when we were crunching. OTOH, I did watch as tired crews would make bad decisions during crunch that ended up costing time in the long run, and I can definitely say that 80 hour weeks are not twice as productive as 40 hour weeks.