IIRC, Sharingan controls the Kyubi's will, not the chakra.. Naruto's new Seal locks up the Kyubi's will, and lets him draw up its Chakra at will.. I believe as a side-effect, he no longer heals on his own, as shown by his apparent 'sprain'.. That being said, the new form is bound to give him more of an advantage.. From what we've seen so far, it gives him a boost in overall stats, but takes away his healing ability.. I'm thinking we'll see lot more of his new powerup.. That, and the fact that his next fight will be his first since the upgrade, I'm betting on Naruto