Yamato said it was 50% complete when Kakashi asked if Naruto perfected the technique during the Kakuzu fight.

Alright, let me retract everything I said about how to estimate the power of the techniques, save for one thing: the rasengan is a ball of MAXIMUM chakra out, spun at MAX speed, made to STAY and RETAIN it's shape with whatever it comes in contact with. That's what the rasengan is. When a regular rasengan came in contact with a chidori, they canceled each other.The chidori can "cut lightning", so one can imagine it's an extremely powerful piercing technique, and an elemental one at that. Yet, a regular rasengan is just barely canceled by it, at the cost of the chidori itself.

Ok, now, lets add the wind element to the ball with staying power. Concentrated chakra as I described it in the previous paragraph, with the wind element all the good stuff that comes with it. I mean, I'm not even sure how to continue this, it seems that obvious. That ball isn't breaking, specially if it's coming against the element that wind trumps.

About Rasengan not killing people, huh? When Jiraiya pushed a rasengan through a tree trunk, it utterly ate right through it. When people throw knives at a tree trunk, it gets stuck in the trunk, not very deeply might I add. People have been killed by a knife thrown at them, a regular one, but rasengan doesn't kill? More accurately, Rasengan HASN'T killed anyone you've seen, except for that very big one Naruto used to kill fake Itachi.

Now, this here is just absurd, Naruto didn't kill Kakuzu, Kakashi did, all Naruto did with the wind Rasengan was destroy every cell in Kakuzu's body? Lets analyse that for a second. There are quite a few types of cells in the body, lets take a look at a few of them. Muscle cells were destroyed. This means, Kakuzu's diapragm was no longer working. Meaning, he couldn't breathe in and out. Couldn't swallow. His heart stopped pumping blood to any of his organs. Not that it would matter if the did, or if kakuzu could still breathe because Red Blood Cells were destroyed, meaning they're no longer serving their oxygen transporting function. Kakuzu's nerve cells were destroyed. He's not moving ever again, as if he could even they weren't destroyed, considering his muscles cells were destroyed. The systems regulated by nerve impulses would no longer work like, oh say heart beating. Oh, we covered that with muscle cells being destroyed. What about breathing, oh, that too was covered with muscles destroyed. Overkill anyone? Then there are the cells that make up the liver, kidney, and other organs. All kinds of crap would start pouring into his system in places they didn't belong. Bile, adrenaline, glycogen and a whole mess of other stuff, none of which would be where it belongs. All types of system failure and shock would be afllicting this guy. He should've been dead instantaneously. You ought to get the point. Kakuzu was plain old dead. Why did Kishimoto do the stupid move of having to make Kakashi finish him off? Just for the sake of continuing to make Naruto look like a baby. That had NOTHING to do with the wind rasengan not killing. Because clearly, it kills.