> Is there a difference for how fast you can listen to the newer more nicely sounding ones or the older more robotic ones?

Yes. The main requirements for the TTS I use is it must be intelligible at very high rates of speed and it must have no perceivable latency (i.e, how long it takes to convert a string of text to audio). This rules out use of almost all voices, since a lot of them are focused on sounding as human as possible, which comes at the expense of being intelligible at high rates. The newer voices also usually don't have low latency.

> Is there a specific voice you are using for 800wpm or it can be any TTS?

I'm using ETI Eloquence. If I switched to another voice capable of being intelligible at ESpeak, I would have to slow down because I'm not used to it and have to train myself to get back to the speeds I'm used to.