tsukiyomi and amaterasu and susano are the key to this fight.
You said Kisame's GBS would take down susano but i ask you exactly how? Yata mirror FTW !! - Edge = Susano ... ITACHI
Tsukiyomi is basically instant, and kisame would have a hard time countering it. But samehada can help him... so i think ain't gonna work.
Edge .. : Kisame!
Amaterasu, kisame can counter it by making walls of water , he can create a lake , so defending himself using water would be no problem. But it would evaporate the water itself, so kisame would have a hard time fighting it.
Edge ... : TIE!!
Also The water dome technique isn't gonna work on itachi.
Itachi can just summon susano !
In sasuke vs mei fight, susano protected sasuke from the concentrated mist.
So same way, itachi's susano can protect him from waterdome,
Then all itachi has to do is amaterasu on kisame...
I say kisame have 30% winning chances.