Mark I have recently done the barrier stats for someone on this forum who kindly did me a favour.
The end result over thousands of races over various distances on various tracks and in varied going gave only a slight disadvantage percentage wise to extreme outside barriers.

The percentage difference over any distance was 1-2% and not even worth worrying about.

What this tells us is that the barrier must suit the horse's style of running.The way the race is run also dictates whether the barrier will be of any consequence. Even over short distances this was true.
For example I would not back a leader from an extreme outside barrier, but would back a swooper.
But remember - there will always be exceptions to any rule, that's what keeps us on our toes!