I've yet to find a study that would validate that claim.

The gains from vim over using the mouse is mainly muscle memory for me.

I can comment out a function/block/line from anywhere in the function without looking at either the screen or the keyboard. And I do it without consious effort.

You probably use keyboard shortcuts to go to the end/start of the line anyway, if you expand that to all the vim movements that's what you gain. Adding composition and macros are just the icing on the cake.

I think this can largely be attributed to preference. I have my hands on the keyboard anyways when I write code, so I find it easier to just move with keys instead of grabbing the mouse. For bigger, espacially vertical movements, I usually grab my mouse, unless I know the line I have to jump to. In that case using the movement keys would be faster as well.