I have no idea about the actual numbers/calculations ... but the short answer is that adding 15 AC to a caster is not going to make a noticeable difference.
Now, to be fair, Shaman/Clerics do get meaningful benefits from enough AC, and so stacking AC on them isn't worthless ... but, even for them, 15 AC will not mean enough of a difference that the player can observe: you'd only see it in parses.
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