I do feel there's a leap in the ability to remain in flow while typing once you're past a certain (high) level of proficiency. Looking at what you're typing really interrupts flow for me, at least when writing prose.

If I have a lot of writing to do, I almost always write longer passages with my eyes either closed or gazing at something other than the screen. When my thought is complete I open my eyes and do a few edits on whatever typos made it through.

I also do almost all of my writing in vim, so I never have to context switch between mouse and keyboard.