[Moon (James)]

In short, is a high end mirror really worth it for deep sky imaging?

Good question, but I don't know if there is an answer since it depends on your personal value judgement.
I use a GSO mirror and my images look ok to me, but I notice people with more expensive mirrors often times take much better images. Of course there are additional factors involved, including the skill of the operator!
I would say have a good look on the internets at all the images taken with GSO mirrors and set your expectation level accordingly.
If you want to go down the premium path, you could consider a Zambuto swap over. This is something I might consider one day too.
Just my 2 cents worth.
James