20AC here and 20 AC there make a huge difference. 20 AC is roughly 200HP.
He will still need more AC since he will be tormented and parasited often in battle. The opposing force knows that if he dies more than likely his party will be easy pickings without full heals. Sorry, that is just my experience. How many times have you been in vent and everyone says "Kill the priest!!!" After the priest goes down, the rest of the party usually follows.
That is also why I chose to carry a Holy Animor in RLB over a Lycaon hammer when I played my INT priest. AC whore and weapon defense in RLB FTW!!!!