Well Itachi did say that fighting kakashi, Kisame would suffer unnecessary injuries. But then again a clone of kisame with 30% chakra forced Gai(who could be much stronger than Kakashi) to open 6 gates in order to defeat the clone. Plus the Killerbee fight, Killerbee was protecting his friends but he was lucky since Kisame was doing an infiltration mission and wasn't sent to capture the 8 tails right away. So a with no distractions and no other side missions, Kisame would win against Kakashi, since Kakashi uses high level techniques that take allot of chakra, he would tire out really fast, and if kumai was used on Kisame, he would just regenerate himself using the stored chakra in Samehada. .