I don't think Hidan is kage lvl, its just that he has unique techniques that makes it difficult to deal with like immortality and killing enemies with some voodoo technique. Once you find out the weaknesses to his techniques like shikamaru did he isnt much of a tough shinobi. Just like the fight with Neji vs the spiderguy. Neji's absolute defence was supposed to be one of his ultimate trumpcard but once he figured out a weakness to that technique Neji became an easier target. Kakuzu on the otherhand shows signs of being kage lvl, being able to use 3-4 nature techniques and he is pretty intelligent. So i wouldnt be surprised if Hidan died or disabled, but they shouldnt be able to kill Kakuzu cuz that would be lame.