Oh, I think Iran would get their asses kicked. Decidedly so. We'd take far heavier losses than against a joke like Iraq, but it would obviously be extremely lopsided. Plus, its far easier to simply destroy a place than to try to take it over and occupy it. Trouble is that it doesn't do anyone any good. Especially if we then decided to "nation build" them. Which is probably worse for them than just killing them...especially considering the strategic resources up for grabs.
We'd be trading a somewhat rational state sponsor of terrorism...for a totally irrational high pressure fountain of terrorism and insanity. War with Iran is just idiotic.