Thank you all for your answers, I decided to purchase it through Steam since I will get for the same price a Mac and PC version. The Steam version is also a Cider port and after testing each version I found out that there are some issues with the Mac version.
My current set up is iMac 3.2 Ghz Intel Core i3 with 12GB ram running Mountain Lion, I disabled the transparent menu bar, the shadows, the 3D dock. The hard drive has been recently "defragged".
For some reason (of course emulation) there are some artifacts that I think are some frame drops. When you pan the camera the movement is not smooth and you can notice some sort of chopping. The game resolution was set to 1600x900 which from certain distance I cannot tell the difference from 1080p at least while playing. It doesn't really matter if I choose low or high effects or limit the frames per second, it doesn't even matter if I set the resolution to 720p, the overal picture looks horrible and the chopping still happens.