Remember how Kisame was call the "Tailed Beast without a Tail" due to his impressive levels of chakra because of his own chakra and the Samehada? How busted do you think he'd be if he had Isobu and could control him? Kisame without Isobu is no joke, his clones were destroying Team Guy and one of them made Guy go up to the 6th gate in base form, he also killed the former wielder of the Samehada who was apparently apart of the best generation of the Seven swordsmen of the Mist, he also gave Bee a real run for his money and made Guy go to the 7th gate without the Samehada. Now imagine if he had the Samehada AND control of Isobu, how busted do you think Kisame would be?