taking 20% less damage is the same thing as having 25% more hp
paladin has 100 HP with shield oath on, warrior has 125 HP with defiance on
they both take a 100 damage hit
paladin mitigates it to 80, survives with 20 HP, 20% of his total
warrior takes the full 100, survives with 25 HP, 20% of his total
the passive mitigation has always been balanced, it's the fact that 5x wrath only gave 15% that caused the gap, coupled with paladin cooldowns...