OK i have finished this a short while ago and I did so on Bootcamp.
I bought the game retail, and it forces you to use U-Play here. I hate u-play.... A crappy poor mans steam...
OK performance wise.....
I'm playing on a MacPro 2.6Ghz Octo core, 32gb ram, HD5870, 750gb Maximus XT HDD/SDD combo running Windows 8
I ran at 1920x1080 with settings mostly on High / Very High with post processing set to Low.
The framerate still fluttered mostly it was smooth but often dipped.
There are problems with Far Cry 3 and AMD chips for a start and you will have to do some adjusting of the settings if you want smooth performance.
Firstly..
Run in DX9, DX11 its defaults has more issues on AMD GPU's including a kind of stuttering when traversing regardless of what resolution or quality settings you try.
However doing so means you wont be able to enable MSAA or some SSAO options.
Secondly the V-Sync has issues again in game so best to leave it set to off and use Catalyst Control Panel to enable V-Sync. You will get better stable peformance.
Thirdly, the new patch that came out and will automatically patch the game when you first launch, gets rid of the option for multi-core rendering, so having a decent CPU doesn't give you too many benefits. Apparently for some when enabled it caused the game to crash, so they just removed it in a patch rather than fixing the issue.
Given the 6750 in your iMac is less powerful than my 5870 I doubt you'll be able to play in anything but low to medium at default resolution.
Disabling post processing completely in cfg file will give you a really good boost in fps, but means you lose all depth of field effects.
So basically go for the PC version if you can put up with spending a good amount of time tinkering and realising your never going to be ever fully satisfied. I tinkered with settings throughout my entire 30 hour campaign...
Yes it could look better than the 360 version, but peformance could be erratic to say the least, and you will have to deal with U-Play unless you get it off Steam....
With the 360 you will get 'consistent' performance and you'll spend your time playing the game rather than the games settings.
If i was to do it again, i'd buy the 360 version....
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I went PC, those detailed textures and amazing environmental effects running on a 2560x1440 monitor with a gtx680, its pure heaven.
Yeah the game runs a lot better on NVidia hardware, especially something as sweet as a GTX680
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Edit
Ubi have a 42 page thread of users experiencing the micro stuttering i mention above, so it seems fairly common.
http://forums.ubi.com/showthread.php/730854-Micro-stuttering