The graphics cards are the same chip - correct but they have been binned according to their performance. It's like a lucky dip with GPU's you may be able to overclock it but you can guarantee the heat will increase.
Overclocking in a laptop. Are you serious? You have just paid close to 3k for a piece of technology with no user serviceable parts. If you fry that GPU, you can kiss the logic board and any other components goodbye also.
And as for warranty - no amount of Applecare will save you there...