Tognazzini wrote a magazine column with all the downsides: overly funny, non-academic, etc. I think Tog meant something like selecting commands from a menu vs using a command line across a range of applications. Anyway, studies like that must be somewhere in Proceedings of CHI, I guess. (Just checked bibliography in "Tog on interface", but nothing seemed to match. Found a comparison of different types of menus, but that's different. But also relevant: I guess most people would say using pop-up menu right at the mouse cursor will be faster than a fixed one at the top of the screen, yet the experiment shows the opposite.)
Mousing implies things are visible and you merely point to them. Keyboard implies things are non-visible and you recall commands from memory. These two must have a principal difference. Many animals use tools: inanimate objects lying around that can be employed for some gain. Yet no animal makes a tool. Making a tool is different from using it because to make a tool one must foresee the need for it. And this implies a mental model of the world and the future, i.e. a very big change compared to simply using a suitable object on the spot. (The simplest "making" could be just carrying an object when there is no immediate need for it, e.g. a sufficiently long distance. Looks very simple and I myself do not know if any animals exhibit such behavior, it seems to be on the fence. It would be telling if they don't.)
I think the difference between mousing and keying is about as big as of using a tool and making a tool. Of course, if we use the same app all day long, then its keys become motor movements, but this skill remains confined to the app.