I don't think you'll need all that much to run GTAV at 60 FPS.
I think you might be underestimating the CPU load the game is likely to have - at least if we're talking about a very high LoD, and everything else pushing draw distance in such a dynamic game.
The world is enormous. On the other hand, it's designed in such a way to run on the old consoles, what with the long highways and mountains separating large areas of density.
It's going to be really interesting to see how it plays out. I do expect the hardware requirements to hold CPU and VRAM demanding settings up to needing pretty beefy specs if they want to keep 60.
However from a more conservative standpoint, I think you're right in that it won't take too much power to get 60fps - if we scale back some key settings that strain the specific system points mentioned above.