Surely Iraq that is not stable:

a) Makes it not a threat to Iran.
b) Puts up the oil price, which makes Iran and the rest of OPEC richer.

An Iraq that is stable:

a) Is not a threat to Iran.
b) Puts up the oil price as it joins the OPEC cartel.

An Iraq that is a stable puppet state:

a) Is a threat to Iran as it may be used as an example/base of operations.
b) Pushes down the oil prices as some of the main users get it at discount prices through their companies that have unusually favourable oil contracts. Which upsets Iran and the rest of OPEC.

Iraq being a poster boy for regime change is not just bad for Iran it would be terrible for the majority of the middle east. It would be even worse if the cost of money and people was less... a cheap change would make even SA look affordable...