If Naruto was unfortunate enough to get caught in the Mange Sharingan, the Kyuubi is likely to freak Itachi out of Naruto's mind. Then again, it's Naruto's mind, not Kyuubi's. But at that instant, i would think the Kyuubi would take over Naruto's body while his mind is occupied and just catch Itachi off guard. He'd just be like, wtf? The mind is gone, but the body's going mental.
Either way, he will be ownd.