I'd save my money if I were you. The G13 frees up more buttons thanks to the joystick that you will never need a mouse with more than the standard 5 buttons. While I don't have the G600 I do have the G700, which has 13 buttons and I never find myself using more than 5 buttons, there is just no need for a mouse with mutliple buttons if you have the G13.

Maybe in future expansions with level cap raised and new abilities added into the game should you consider getting a Naga/G600 MMO style mouse.