As much as I would like for this to be true, I don't see this as being true. Reiterating what others have mentioned:
  • The extra 40 shaders don't fit with their current cluster size. And, the Shader/TMU ratio would be of-set.
    In order to get those kinds of core clocks, they would have needed to go back, pick apart the architecture and try and optimize current leaks.
    512mb? If they really don't think that 1GB isn't very important, then why would they even bother creating an official 4870 1GB version?

Just a few quick points, but unfortunatly, I doubt this as truth.