Improve Perion's Mithril Plate Mail
Currently it's inferior to dragon scale mails and EPM.
There is only one real artifact armour in the game - Perion's. (Apart from the chaos knight one, which I don't count.) The rest are robes and a leather armour. Unfortunately, Perion's is also arguably the most boring artifact in the entire game, completely lacking any character or desirability.
With [-1, +13] it's less desirable than a regular eternium plate mail, or the semi-abundant dragon scale mails, which grant immunities. With the addition of full plate armours, where an eternium FPA is around [-3, +18], I suggest that this only true artifact armour would be buffed a bit. For example, [-1, +20].
It's still a very rare item and +20 is still far from the hulking armours and anything else so in my opinion this would not be imbalancing.