I'm not a specialist but maybe, check your video driver settings (not in the game but in your computer). On my pc + nvidia, I need to configure the driver for "use game settings only" for this game or I have some bugs. Maybe your driver force high antialiasing or other options that use more power.
I don't know if the Intel Core i7 on MacBook has an integrated gpu (Intel HD Graphics) like on pc. It's possible that the game uses this and not your ati, and that could explain why it's so hot. In this case you can try to disable this driver video card (I don't know if you can force the game to use the ati)
Hope this helps
