It seems to me that a current key tenet of neoconservatism is that Iran is on the verge of revolution, that a revolution will decrease the chance of Iran causing nuclear problems, and that we can push Iran to a revolution with minimal expenditure of treasure and no expenditure of (American) blood.
Further, the assumption is that because the insurgency is supposedly mostly foreign fighters, or else is dependent on foreign support, therefore taking down Iran will quiet things down in Iraq and (presumably) allow us to bring some troops home, thus alleviating the army's recruiting problems.
Taking down Iran will also - in theory - take down Syria, thereby reducing two of the states that are presumably the biggest sponsors of the insurgency.
In theory, we can expand the war without expanding (and in fact reducing) our troop commitment.
Unfortunately, I think that if we actually try to instill unrest in Iran, we will be more likely to find that we either get a lot of dissidents killed, turn a lot of dissidents against the US, or wind up being forced into a ground war in Iran. Probably all three.
In any case, I don't think we can expand the war without having to expand the actual ground war, which we can't do without either a draft or resorting to keeping order through mass murder (20,000 soldiers could probably keep order in Iraq if we were willing to resort to shooting 500 Iraqis every time a coalition soldier was killed). Whether the neocons who advocate this policy believe this as well and are lying to us, or whether they are actually deluded themselves I don't know, although I suspect that there are some in each camp (i.e. some liars, some naifs).
That is all.