If I could type that fast, I would immediately try to use the keyboard to think on screen, like thinking out loud (or subvocalizing).

Writing as I think is like climbing with a rope. I don't get mental superpowers, but I do have an easier time mapping out ideas, as I don't have to rely so much on memory. Then faster typing means faster, easier ideation. Maybe I would journal more.

It's not about throughput, it's about latency. I want to get fast feedback on my last thought, to take into account while forming my next one.

It's the whole waterfall-vs-agile on a micro scale.

Same, even when bashing out an HN post I spend more time thinking than typing. When coding I spend far, far more time thinking about it (and the better I get at coding, it seems, the more this ration swings towards thinking and away from typing!)

Agreed that composition time is largely made up of thinking, however once you've decided what you want to write, you still need to write it.

The other thing that I sometimes end up doing with stenography is just a stream-of-consciousness-style writing where I then have to edit it down.

On a computer yes. I think there's potential for chording to be a decent input method for a phone or AR display.