Gandalf would turn in his grave...

The way it should be, from a DPS perspective:
dual-wield caster 1h > staff > caster 1h with shield

I don't think the problem is that caster 1hs are too powerful, nor that caster staves are (excessively) too weak. I think the problem is that caster shields add too much DPS to builds which already have a strong intrinsic motivation to use shields anyway.

Outside of the implicit mod (which is fine), you shouldn't be able to get spell damage on shields. Also, staves should have special suffixes for better cast speed and spell critical strike chance.