Okay here's the facts. (damn i havent been on for a while)

Mangeyou Sharingan is a mental attack, HE controls everything in "that world" and can do anything he wished, if he wanted to kill someone and make him one ounce he can. Thus if even if you don't look into his eyes, it forces you to not read his full body motions and predict an attack, thus leaving you helpless.

O <----=Body needed to predict the movments and attack accordingly, just knowing the feet paterns
-|- <----/makes it difficult to find out where his punches, kunais, jutsu's and ect are going to launch from
/\

Naruto's Kyuubi powers may not stop itachi, since once you see the attack, you can really just say your dead, Mentaly the Fox does not control or aid Naruto, Physicaly he gives naruto Charka.

Lets put Itachi's Full power into effect. Since when he was battling Kakashi, Itachi didn't unleash anything beside the Mangeyou Sharingan. As kakashi said "Becarful, he became the leader for anbu at 13!" "No, he's not even showing his true powers yet" Or something along those lines. And Orochimaru said that even Itachi is stronger.

Mentaly Kyubi is helpless, so what if it unleashes all its power? Once the mangeyou sharingan locks its done. But if the Kyubi's powers unleash FULL before the lock on, then there could be a different question. But since for naruto this is still impossible, since he cannot even unleash 50% of Charka because the body does not accept the high ammounts of charka and cannot handle it (From infering of how powerful kyubi was and the talking of Jiraya) Thus Naruto would die instantly if he didnt know what hit him. The key to killing naruto is to go in fast before Kyubi awakens.

As for Gaara, he would be dead before he unleased his Full Form.