This is a very specific gearset then. I've tested many different gearsets and in all of those I found mastery>crit.It seems, to my Simc results, that if you are below a certain crit rating, crit will value higher even with t17 4pc.Single target: int > spellpower > crit > mastery > haste > multistrike > versatility
Single target with 4pc t17: int > spellpower > mastery > crit > haste > multistrike > versatility
About 1650 crit (without incineration) for me, just swaped arround some gems for testing.
gear was Highmaul Mythic Gear (Troll) with Head, Shoulders, Legs and Hands Heroic Set.
Keep in mind this was before the announced 6.1 4pc t17 nerf. This will probably shift crit above mastery for some gearsets.