I've worked remotely for about 5 years now. Without the water cooler talk, chit chat, and misc distractions, I can easily fit a 8+ hour day into less than 6 hours when I work from home.
I’ve worked with new hires that literally take entire days off and give excuses why they were unreachable. Sadly they are just shuffled around the company instead of disciplined but that’s another topic. But I figure in that case, they were probably not so great workers in-office either.
But I can't really imagine how I would have done well working remotely towards the beginning of my career. It was a different time with far fewer communication tools but, even taking that out, it would have been very difficult both socially and in terms of work discipline.
I suppose that could be one measure of productivity. Another view would be an increase in the number of backlog items closed in the 8hour.