You're completely right about the Itachi and Sasuke cases.
But I disagree with your point of view in the Kisame subject. It's true that he fought against Guy's team while using only 1/3 of his chakra, but Guy's team is in a completely diferent league from that of Sasuke, Naruto, orochimaru, jiraya, Itachi, kisame..., etc. Any of these strong guys can kill Guy's team within seconds. The only one in Konoha, besides Naruto and Tsunade, that could be on this level is Kakashi, and he would be weak.
The thing is that there is no merit in using Samehada, because the sword is the one that does everything by its own..., Kisame has little to do with samehada's performance during the fight..., he just has to wave the sword a little and let it do its thing. On the other hand, KB has a lot of merit for being able to control the Hachibi in the way he does..., and we know that because we've seen how hard it is to do so, when Naruto has to handle the Kyubi.
And by the way, Killer Bee didn't go out in full force against Kisame during the fight because he didn't want to harm his master and it's pet, and even though, he almost kill Kisame when he crashed his chest...., and again, Samehada saved the day.
I couldn't of said it better myself people were to blinded by kisame for some reason.