Literally the second Kakuzu went seriousin that fight, Kakashi was dead, and Shikamaru's plan kicked out. If it hadn't, Kakashi'd have lost his heart. Hidan is not kage level, and he's lackluster at the side of other akatsuki members i give you that... But Kakuzu isn't. Feat wise, he's at the same height Sasori is, who got taken down by a lowkage (At best, chiyo) and a high chunnin level. Even if he let himself die, he still was forced to that point. So i don't see what's the problem with Kakuzu here. He shits on zombie arc Kakashi had him been alone, so he's at least a tier above that Kakashi.
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Here Kakuzu goes serious
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Kakashi notes it was "fast" so Kakuzu definitely powered up in that moment, since he obviously gained more range because of his threads, and the speed of his attacks hadn't been hyped like that before. Kakashi being tired doesn't mean he was "exhausted" because he even used a sunshin and a raikiri after than, and was thinking on uing Kamui (which by that time implied at least 30% of his chakra reserves) so he could have dodged if he had time to.
Kakuzu without going all out forced Kakashi and company to that point, and the second he went all out, the battle was over for Kakashi. He's pretty much above that tier, and he's a lot stronger than Hidan.
And Kakuzu and Hidan also
easely took down Yugito (show by the fact that Kakuzu was sitting on his ass and had no injury at all, and showed no kind of physicall pain or strain), so they were a lot above her as well.