Thank you so much for the advice!1. I would suggest picking up fire for mythic ghuun. You can be arcane for every other boss without problems.
2. Mages are quite important in mythic, on specific bosses. For example, on Fetid Arcane is one of, if not the, best spec for dealing with the small eggs out of all classes. Besides that, every raid needs at least one mage for Arcane Brilliance
3. See above
4. I think arcane is relatively punishing, and so is fire's combustion window, but outside of that, I don't think the skill cap is extremely high for mages. Compared to other classes, I'd say we're about middle of the pack
5. Definitely, mostly thanks to the 5% buff. I wouldn't run it on Fetid or Zul (because cleave can hurt the raid a lot there if running many rogues), but all other bosses fire is quite good. The last 2 bosses specifically I would say fire is the best mage spec.
Just a slight follow-up to #1. My mage is only ~360ish at the moment, would it be fine for me to be fire (instead of arcane) for all bosses, except those you excluded in #5 (fetid and zul)? Or would arcane still perform better overall until we get the 5% buff?
Edit: Didn't know 5% buff is already live