Would fire outperform arcane on thok? (even with the huge helm downgrade?)
Fire looks like it is more consistently performing higher on thok. Also, if you are going to be doing siegecrafter, you're going to want to be fire for that, and likely for Garrosh as well.
Thok/Siegecrafter/Garrosh overall fire is going to be more helpful due to significantly better mobile dps.
May depend on your strategy. If you are trying to keep him in phase 1 for super long periods of time, fire is going to win for sure for dps on thok himself, with all the instants you'll be able to get off on him. Arcane will be better for AoE on the adds. if add dps is more of an issue.
My official opinion for you is are you going to be working on siegecrafter after thok, and if that is the case, I'd say going fire now would be beneficial in the long haul. Specifically for thok, the plusses and minuses of fire v. arcane are probably minimal, since your personal dps on thok seems pretty good.