Is it possible to create a real time game with node.js that requires twitch reflexes. How high is the latency? How low can it realistically go?
It's possible, but it depends on how much data must be transmitted between server and client and how fast (speaking of latency). Take a look at Sousaball by Creationix, for example.
Also, if you plan to use websockets, take a look at Socket.IO library by learnboost. It uses websockets when available and falls back to comet in other cases.