Let's say doubling your holidays means roughly 10% reduction in work time. If a fully dedicated 37.5 h person creates one job per year, could you estimate how many jobs creates a) fully dedicated 41 h person b) fully dedicated 34 h person?

Of course, people are different and this is just an anecdote, but from my personal experience (a couple of instances) I can push myself to "fully dedicate" more than 4 hours a day of mentally really hard work - for some months. After that I tell politely that sorry, it is time for me to do something else for a while. More often than not, I have been welcomed back later. Of course, those have been exceptional circumstances, typically there is no need to fully dedicate more than 4 hours of mentally really hard work, but there is some less demanding aspects of the job taking some time off.