It's a more-than-linear improvement over 40 hours, because when pro-rated you have a lower density of context switches, getting-back-up-to-speed, etc, as happen in the morning or when you eat or such.
Maybe too many people have done long-term damage to their attention spans via the internet? I dunno.
At the risk of being mildly provocative ... are any of the people you know, who can't do more than 40 hours of work in a week, world-class in their field? If not, maybe there is a causal link between these two things?
I'm sure these 80 hour continuous productivity guru's exist. I'm just saying they are so exceedingly rare to effectively be unicorns.