Good question; I had briefly considered whether "better focus" was actually measurable, then forgot to think about it further.

So now I've thought about it a little further and (maybe there's a bias name for this phenomenon, but) yes, I will be going with subjective effects. It's not clear to me if "focus" has more content than feeling focused, and in either case, what I want is the feeling of being focused -- i.e., an awareness that what's going on in my head corresponds closely to what my memory and senses tell me is going on outside of my head.