And....it doesn't affect tone as much as people think. You do get a little better string response, but it's mostly cosmetic.
I'd have to go with 100% cosmetic.
I had this argument with a classmate when I was in college studying electronics and I could hear no difference, and he insisted that the E strings were clearer/louder on F spaced.
So, we plugged a guitar into an o-scope, used a small weighted hammer (on a hinge) to strike the strings for consistency... then replaced the pickup with the same type but F-spaced and did the same test... and except for what you would expect the variance to be between manufacturing, the E-strings were no better balanced on the F-spaced compared to the regular spacing.
Of course this was one test done with only one guitar and two pickups... but it was good enough for me.
I think those that say they can hear the difference are simply victims of suggestion... but that being said: I do believe that if you think
you're hearing something, even if it not truly there; you're still hearing it... because your mind processes it as such.
It's like those of us that get flu symptoms after a flu shot. Sometimes it's your body reacting to the shot, other times it's psychological... in either case; you still feel like you have the flu so it's the same difference.