Kisame's chackra was always put on the same level as a Bijuu. We just hadn't really seen what he could truly do.

The thing that bothers me about Samehada is... if its the sword of one of the 7 Swordsmen of the Mist... how come Zabuza's was so... simple? Was Zabuza the lowest rank on them? Or did Kisame infuse his sword with a demon or something after leaving?