There is really no super spec. Arcane does good ST and good AOE but it has to be more in melee range and manage the rotation harder, Fire is good at AOE and barely mediocre in ST but generally easy to play while Frost is best at ST, not as good in AOE. In contrast Arcane has the fewest keypresses a second.
Normally, most minmaxers will swing between spec 1 and spec 2 and sometimes even 3 but now with the legendaries, AP and other gearing it's less so.
My personal opinion is that very few minmaxers and very few casual players will switch spec this entire expansion.
I'm basing myself on the logs, the difference is huge. Arcane is not an option because I hate the game style, my doubt is still frost and fire, I like frost but I feel the dps is weak compared to fire.
We have to be careful this expansion when checking logs. In other expansions a nerf/buff cycle of patches like the last few weeks' would see almost all minmaxers switching spec and many casual players following them, within 4-10 days in total. Right now most players, especially those that started at the launch, are soft-locked into the spec they started with because of spec-centric legendaries, AP and other gearing so you can't tell exactly if Fire is as good as it looks. My personal opinion is that it would look much worse if we didn't have the Legendaries and Artifact system because almost all minmaxers would try Frost or Arcane way much more than they do now.
Also almost no one (literally) played frost at the launch so few legendaries of frost are around.
A large part of spec switches will happen from new mages instead of those from the launch.