Hey man you should know that the Kyubi Chakra Mode and Bijuu Mode are different. The Kyubi Chakra Mode is a form that Naruto gained after he separated and sealed Kurama from its chakra. He could access its chakra directly whenever he needed it without interacting with the fox at all. In doing so, he gained a chakra shroud that resembles the Sage of the Six Paths' silhouette. Later while fighting Tobi's Six Paths, Naruto reconciled with Kurama and gained access to its full powers. When they meld their chakra together, he gained Bijuu Mode. Bijuu Mode is way stronger than the Kyubi mode. Your way of viewing power is correct in terms of physics. But the word power has different meanings. But let's not talk about it. Let's talk about what u said. You consider power to be brute strength and even gave an equation. But that's not how we view power you know, not at least in fantasy world. Like Saminyt said, power is the sum total of strength, speed, stamina, intelligence, technique and chakra. If someone was to consider someone powerful only because of brute strength then they would say, "Hey you know Tsunade is way more powerful than Jiraya and Orochimru coz she got this awesome brute strength. She can crack huge boulders with a punch while neither Jiraya nor Orochi can do it." This is what they would say. Not only that, they would even consider Sakura stronger than Sasuke because of her awesome strength she inherited from the 5th.