I think the current stenotype techniques approach the fastest keyboard-type input you can have. At 300 WPM+ I believe it gets tough to perceive or come up with words fast enough to type them. 
(I seem to remember that a fast speaker approaches 300 WPM whereas a fast head can easily think around 800 WPM - however, this kind of fast word-thinking is probably somewhat non-verbal so I don't know if it's a fair comparison. People who "read" 2000 WPM certainly don't enter the equation because they're apparently really taking mental snapshots of the pages and not reading them through word by word.)

(I seem to remember that a fast speaker approaches 300 WPM whereas a fast head can easily think around 800 WPM - however, this kind of fast word-thinking is probably somewhat non-verbal so I don't know if it's a fair comparison. People who "read" 2000 WPM certainly don't enter the equation because they're apparently really taking mental snapshots of the pages and not reading them through word by word.)