I agree. And you seem to understand it correctly. It would be OP in 1v1 of the same class. But 1v1, the game is horribly unbalanced. 1v1, it's basically just a game of rock-paper-scissors where one class always beats another, and a warrior priest would destroy almost any other class (other than a desciple of khaine, which is the chaos carbon copy of a warrior priest, or a witch elf, which specializes in surprise burst damage that I might not be able to outheal in time without Morale abilities which only build up over the course of sustained combat) because they're a melee DPS/healing hybrid, who can literally damage, heal, and heal as a result of doing damage.
I played Warrior Priest for almost my entire Warhammer Online career. I focused primarily on support, but in 1v1 I could pretty much outlast anything that couldn't stunlock me (again - Witch Elf). The neat thing is if the witch elf botched the stun, I could detaunt her and then outheal her damage pretty easily.
That was another thing I liked about Warhammer Online... taunts and "de-taunts" had PvP use... in that taunting a target made it do bonus damage to you but reduced damage against anyone else, while de-taunting would cut their damage against you in half but wore off immediately if you attacked them.
But the thing I loved most about Warhammer Online was that PvE was entirely optional. So many times I leveled from 1 to max level purely in PvP scenarios and battlefields. I leveled and deleted something like over 50 warrior priests, because PvP (or RvR as they call it) was fun, and I hated PvE grinding. And the game also made slain players drop generated gear and loot currency (so that you got equipment rewards for winning in PvP but didn't lose any gear if you lost in PvP), and this PvP-generated gear was on par with any PvE-attained gear, so you could deck yourself out by killing other players just as easily, if not more easily, than doing traditional PvE.
Aaand boy did I go off on a rambling tangent.