Your numbers are basically correct. I seriously doubt that you're going to experience much improvement. Perhaps, but not likely. You didn't mention that your current setup makes it impossible to do this or that type of analysis due to the lack of resolution, so it seems to me that strictly for this purpose, a D800 would be an unnecessary expenditure.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!