Well, I really like this hypothesis of ISIS plotting to overrun Iran. Instead of “my enemy’s enemy is my friend” I prefer “my enemy’s enemy is my enemy”. I say let them have at it.
Personally, I think that they will capture the rest of the territory in Bilad-al-Sham before moving on to Persia.
That's not to say that they won't try to capture Iran's nukes...but that is different than capturing and holding territory.