Well, as the matter of fact, one of these people is a compiler specialist and the other one is the most senior technical person in his company. We all agree from personal experience that not doing more than that amount of work works the best in practice. We've been in this field 10-15 years, each.
I have personal experience that working about 30 hours a week works well. I am energetic, my thinking flows well and I get lots of stuff done. I have also worked myself to the ground by attempting to do more. That doesn't work and my experience has been mirrored by other experienced people who have tried.
If you still believe in measuring in this field, you have a long way to go. There's no good measure of amount X of work done in time Y, because it cannot really be compared w/other work unless you're doing basically the same exact thing twice. And we mostly don't.
The bottom line is that there's things we can't really measure effectively. I know that this drives the managers mad. However, I can only tell them what works and what doesn't. What does is that you remove all the obstacles from doing only the real work and minimize task switching. Then let the people do that for 30 effective hours a week. That how I'd basically run a software company. Of course there's more to it, but that's out of the scope of this discussion.
Above all, however, do not take my word for this. Go and find out yourself. Trial and error, adapt and improve.