Wait... you don't get the domain ability along with the spells, do you!?
I'm under the impression that Arcane Disciple only places the domain spells on your class list, such that a wizard could then scribe them into his spellsbook, a warmage could cast them as any other warmage spell (but the feat needs to be taken at creation, because the warmage's spells are inherent knowledge from brain-washing-esque training at their college), and a sorcerer could take them as spells known, as per his normal rules.
The domain ABILITY, however, is only for clerics. Otherwise, yes, this feat would be quite a bit overpowered.