Suigetsu seems to be compared to Kimimaro, who for me is not on the same level as the legendary jōnin (including Sasuke, Kimimaro and Orochimaru). Kimimaro, if a legendary jōnin taijutsu specialist, should have defeated Lee in one hit. That assumption comes from seeing high-level shinobi of that level handle low jōnin in one hit (Asuma against Sound shinobi, for instance. I cannot believe that Kimimaro was close to the highest levels, and see him as mid-jōnin.
I don't think Suigetsu is likely to be much more powerful than Kimimaro, though high jōnin is possible and a trick or two may allow him to have a decent fight with a legendary, at least for a while. But his desire to fight Kisame doesn't mean he's necessarily on that level. Look at what happened when he pissed Sasuke off: Sasuke moved so fast that he was between Suigetsu and his target (Jūgo) faster than he could react, in which time he caught both of them with sen'eijashu and paralysed them with genjutsu.
Puts a rather cruel limit on Suigetsu's maximum level.