please prove this. i am now committed to wfh, yet to set and reach important milestones i have to buckle down and focus in the office.

Many very productive people will not work in an office at all, thus harming productivity. When you're remote, you can hire the best.

I would also be interested to know how true this is for individual productivity versus group productivity.

There is no way to prove this and you know it. However I can tell you I'm orders of magnitude happier working from home, and that makes me a better employee overall. I'm not driven by stress and resentment, but actual will to improve things and deliver.