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Actually, way back when PalmOS and PocketPC were all the rage, there was a company that released something called the Fitaly keyboard. I don't know what you qualify as "great" typing speed, but with this keyboard they advertise 50wpm with a stylus, I was able to achieve 60wpm after some practice.
The point is, it is definitely possible to achieve fast input speeds on mobile devices.
Admittedly it did take some practice and probably isn't great for the average consumer.
I never got up to 60wpm with Fitaly but I do remember not really feeling hampered by my typing speed.
In general you don't get a stylus with your phone anymore these days though...
Also it seems like it's finally coming full circle around back to styli again. It seems like they're now being advertised as a new feature, which I think is kind of funny. Granted, they aren't using resistive screens anymore, so it is sort of new.
I'm waiting for a good stylus-enabled device before I purchase anything new. The fact that you can't draw on most of these tablets is silly.
In the case of a false positive (correction where none was needed) you can just delete the entire word and type again.
As this is usually done using Markov chains which learn from your previous input this could even be used for programming.
For the desktop this might be quite amazing however since, unless you're a perfect touch typist, you will always make a small number of mistakes.
These small mistakes can be caught fairly easily (especially given the much larger training set you'll gather on a desktop) and since you're looking at the screen anyhow it's fairly easy to detect damn-you-autocorrect mistakes before they slip through.
TL;DR; Don't use this to make inherently slow data entry a bit faster; use it to make fast data entry even faster.