It would never work, you cannot manipulate demographics like that. Sunnis and Shias are not living in seperate areas, although some regions are predominantly sunnis, and others Shias, but you cannot seperate them into different geographical parts.
the partition plan has been tried in lebanon, it has been tried in Palestine, and it failed miserably in both cases.
I can't believe im saying this but the the US are in too deep to quit now. withdrawing us troops would be the worst possible mistake at this point (an open invitation for Iran to take full control).
what lacks in Iran is a clear strategy fo the US, cuz i don't really see what they are trying to do (unless they are encouraging a civil war...)
If the aim is to counter Iran expansion, they have to ally with the sunnis.
If the aim is to secure Iraq, than ally with Shias
If the aim is democracy.. tough luck, too late for that. Should have thought about that earlier