He would still be pretty good without samehada, but he would be even better with it. He was called the tailless tailed beast for a reason, he had enormous chakra reserves even without samehada. And he could still absorb charkra without samehada (ask Aoba Yamashiro how he found out). Kisame did manage to kill fuguki to obtain samehada in the first place (a seven swordsmen member). The fight with gai was just a bad matchup on kisame's part. Kisame was already exhausted and resorted to a final technique to end the fight quick so he could return to akatsuki's hideout. I'm sure if his mission was to destroy or capture gai, the story could've been different (the killer bee fight would be a good example).