I think it's a little too fast. That one scene when Kurama cheered Naruto on after unlocking more chakra for him to use felt completely out of character; even if he really hated Madara more than anything, he should at least hope that they kill each other, not gleefully hope that Naruto would win, as it appeared to me he did. All of a sudden Kurama went from a stubborn ball of hatred into a sappy little puppy dog hoping the best for his master.
To me that came out of nowhere and suddenly jumped their relationship into definite BFFs to be, all in a matter of a few days. Sure we were just shown a flashback with him reminiscing over Naruto's growth, but if he really cared so much about it, we would have seen changes in his character over time, not him trying to corrupt/kill Naruto at every turn up until very recently.