I usually only lurk on the MobileRead forums but I signed up to share the interesting experience I've been having reading hundreds of books at gradually increasing text to speech rate. It turns out if you put yourself through a kind of training regimen you bump into the upper limit of speed supported by modern text to speech engines before you bump into the upper limit that your brain can be made to handle. Plus, there seems to be an unexpected intelligence amplification effect.
http://www.turnkeylinux.org/blog/int...text-to-speech
It's a bit counterintuitive but I'm hoping there might be a few eBook fans on this forum willing to give it a try and see for themselves.