I've often felt that many imagers are trying to use too long a focal length guidescope when trying to autoguide. I've never had anything to prove this except in practical evidence. I have far less guiding problems using a 200mm finderscope than I ever had using a 420mm guidescope. And this is imaging at 1800 - 2000 mm imaging scope FL across a variety of mounts.
This article though seems to go along way to convince me I'm right. Again I can't prove what he is saying, but it makes sense to me and the evidence I've seen with my own gear tends to support his ideas.
http://www.wilmslowastro.com/tips/autoguiding.htm