To break from the Ifrit idea, and even NMs which are also "tuned" to be more difficult than equally leveled mobs.

Find yourself on a level 40 job, now w/o changing gear between steps:
Let a lvl 52 mob hit ya
Switch to a 45 job same mob hitting you
then to a 50 job w/ the same mob
Always in the same gear, you change in stats should be minimal.
See how much impact dLvl still has even now.

I missed a step in this during this process but when a friend and I were doing leves we set 52 as our target lvl
went out I was 42PLD, I got nailed for upwards of 1k/hit.
Switched to MNK50 avg. hit was ~80
My def didn't change all that much what w/ MNK being in not uber def gear and PLD having +vit & hi def gear. I'd check it but I'm at work and can't.
Either way, 8 levels was the difference between being nearly 2shotted on PLD and almost completely ignoring the dmg I was taking on MNK, I only took into acct AAs since my PLD couldn't survive to experience a WS but I had PLD buffs up during it.
I hope this shield mechanics change helps more than a little.