i have a late 2013 maxed out 27" (780m). I play with high details and native 1440p. the problem is (like with all games on imac), that any resolution lower than native looks totally crappy. currently it is a little laggy but thats because the poor optimization of the alpha build... hopefully it will run much better in future releases... but dont get me wrong.. it runs pretty well!
i love this game! try it!
edit: try MSI Afterburner and win8... i think win8 has a little more gaming performance.