Solid point, but there is no reason to believe that Sasori's supply of poison gas will come even remotely close to rivaling the amount of mist Kisame can emit. It will be released, and then eventually dispersed into nothing... Especially when we factor in Deidara's clay explosions, which would be even more effective then Sakura's explosive tag [
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]. All this, and in addition to the fact that the real Kisame and Deidara wouldn't be flying within the oppositions vicinity because, A) Full intel makes them fully aware of the poison gas possibility, B) Letting off C4 within the mist would be a logical strategy and a priority, and C) Kisame will know of their exact location granted Samehada's sensing capabilites. And I do not see how Mizu Bunshins (composed of water and chakra) will be effected by the poison gas, assuming it even reaches them. Also, any explosion or effort to disperse Kisame's mist is ultimately futile. Neither Itachi or Sasori are contesting him in chakra capacity; he'd simply emit more.
Susano'o will always be trumped by GSB, so that is hardily a factor in this fight. Unless you'd want to argue that Itachi maintains a V4 Susano'o equipped with Yata Mirror throughout the entire duration..? In which case, is still futile when we factor in the mist, full intel, and Samehadas sensing. An undetected C4 would GG Itachi at any moment his susano'o is not activated. An undetected C3 (enhanced with Kisame's chakra) being dropped on the team more then likely GG's them as well. Your previous strategy hardly takes anything Kisame and Deidara have to offer into consideration, and is even less viable under the Hidden Mist conditions. Totsuka Blades databook description makes it quite clear that it only seals living entities within it's genjutsu.