(Or, if this evidence is anecdotal or otherwise not easily linkable — please do elaborate!)
Most of it is anecdotal. The way I learned to touchtype was participating in chatrooms when I was younger; if you took too long to write sentences, the conversation would pass you by. So I quickly learned to type more quickly than I could talk. A more efficient way to learn is a blank keyboard. Here is an expensive one, or you can buy stickers for your current keyboard for $2 on Amazon, which also lets you learn letters one by one.
How would my typing speed affect the fact that at some given moment I need to read several pages of documentation, sketch out a UI layout, look through code, think, etc.?
The sort of activities you engage in will depend to some degree on the costs of those activities. If you can't type quickly, you're unlikely to participate in chatrooms or irc channels. The amount of journaling I do, say, might depend on whether I write my journal with a pen or with a keyboard, because it takes me far less time to press a key than to form a letter. If it takes fifteen minutes to jot down my record of the day rather than an hour, that might be enough to move the habit from not worthwhile to worthwhile.
I don't know what an "effort model" is, but I take from your comment that if I am not concerned about RSIs, Dvorak etc. should not interest me. Confirm/deny?
Confirm. For me personally, it wasn't worth the investment to switch from QWERTY to QGMLWY because transferring capped my typing speed at 7 wpm at a week, and the adaptation period typically runs ~2 months, suggesting I would be mostly out of commission for much longer than I thought was reasonable.
An effort model is an estimate of how much energy it takes / strain it puts on your fingers to press the key. Some fingers are stronger than others, and "home row" keys are easier to press than keys that require movement. (I move my hands around the keyboard, and so my "home row" is actually on several keyboard rows simultaneously, and moves based on what sentence I'm about to write, so the actual effort model for me is much more complicated than something like carpalx's.)
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.