I spend a lot of time on email, and I would be able to get more done faster if I could type faster/more accurately. I've also tried using TTS on my Mac, but I find it doesn't work well enough (mostly it's a pain to make corrections) to justify the switch.
I think even highly skilled typists usually don't exceed 130wpm or so, except for competitive typists that are copying texts. Maybe changing your writing style could give better gains?