I feel like the kyuubi's power is completely underrated in the majority of part 2. Think back to the very first arc, zabuza and Haku, the first time we see the nine tails' power. Kakashi, one of the more powerful characters in the series stated that Haku was possibly even more powerful than himself (at that time). Once naruto tapped into the nine tails' chakra (not even a full cloak) he completely fodderized Haku. Haku couldn't keep up with his speed or power, and was beaten in seconds. This was with a minute fraction of the nine tails' power. The next time major time we saw it was naruto vs sasuke where he finally went one tail cloak. He was dead even with second stage curse mark sasuke 3 tomoe sharingan. This was with one tail. That seemed like a reasonable fraction of the nine tails power, the demon fox that destroyed konoha. In part 2 we see nine tails cloak against Orochimaru and pein. He wrecked both of them. Now he has finally mastered the nine tails' chakra and seems much less powerful fighting in the war. Granted his enemies are stronger, but considering his strength against Haku and the fact that all minato could do to stop the kyuubi itself was sacrifice himself with RDS, I feel like kyuubi mode, let alone bijuu mode, is entirely underrated. Naruto seems barely stronger if at all than he did in sage mode.