I believe Sayid is Sayid. That's not to say something strange or special hasn't happened to him, but that I don't think MiB or Jacob is inhabiting his body.
Jacob hasn't been shown to inhabit dead bodies thus far, and the show seems to have a device for Jacob to comminicate to people through Hurley. Although MiB has been shown to mimic dead people, he doesn't actually inhabit their dead body, he simply appears in their form. In the case of FLocke, Lockes dead body is still on the beach, dead and cold. MiB isnt actually inside it.
I think something is being slightly overlooked with Sayid. I think the most significant thing with the whole Sayid story is the colour of the water. It would appear that Sayid is the first person to have gone through this ritual whilst the water was discoloured. For better or for worse, this is what makes Sayids experience in the temple historically unique.
We only know so far that Ben has been through that experience at the temple. Perhaps all 'the others' have been through it though. I find it incredibly interesting that Sayid is the only one to go through this in dark water.