This is 15 AC on a caster range item, with 2 int and 20 mana. The next easiest to get (enchanter) range item with 15+ AC is the Divine Inspiration from Maidens End, which is obtainable at more like 55. 2 int and 20 mana is also not bad for a range item at 45 (I had a Sarnak Ceremonial Dagger / Seal of Thoridian at that level ish)
I am not saying its uber, AC isn't all that important to a caster, I'm just saying its balanced.