Just a small thing, but if you are playing sejuani with maxing W you lose a LOT of ganking power. The power of maxing E first is huge. W does let you clear stuff faster if you choose but if you want to have more successful ganks, then the extra damage from more levels in E will help you out more than W.

I am of the opinion that every mastery observed separately looks like useless (if there is such a term at all). Masteries are made to give tiny "peculiarities" or let's say "stat changes" so that even the same champion can differ from summoner to summoner, and their selection to define your direction in both role and position. That being said, they are much more useful to me than other masteries, in the way I play seju.
As far as quints are concerned, first of all Sejuani is not Hecarim to have definite advantage in taking movement quints and in addition I believe that with my proposed build any extra movement, especially over tankiness, is a waste.

Masteries like Enchanted Armor or Perseverance are useless and Greater Quintessence of Percent Health is inferior to something like Greater Quintessence of Movement Speed and so on.