I personaly dont know what to tell you. As far as my information about mages goes in 5.4 all 3 specs are almost eaqual in same gear or atleast in your exaple with 570ish ilvl with Arcane have a slight edge over the other two.
Bu i also have to add that SoO encounters are made so every spec and class have its niche, ups and downs, so thats why you cant find any straight answer witch spec is FotM.
I think that Arcane is that spec (well i will alwyas think like that becaseu i love it
), but yet again that depend really on your raid size and composition and what are your "duties" during the fights. For HC progression i saw many people say that Fire is THE spec because of the mobility it offers. And for Frost i havent seen much, but that mostly because i dont care about that spec so i cant say anything about it