You picked the three dps classes I don't play so it maybe the case but for everyone else swapping out gear and enchants tends to be at least a 5% increase...
Mythic dungeons also tend to heavily favor certain specs depending on the keystone. You are not running heavy aoe or cleave with skittish for example.
I am not actually sure how tier sets are going to work they played with the idea of lfr having the same tier before scrapping it but I don't know why you expect raiders to not have some kind of gear advantage in pve...