3rd time trying to post to this ftw...
1- When you say you're selecting the Ultra setting and had issues, did you try clicking the button on the bottom left corner that says 'Recommended'? I can't remember if Blizz ever fixed some of the issues with certain settings conflicting with one another or not. They tend to be ridiculously slow fixing things this xpac. If you haven't tried the default recommended button, try that and see how it is.
2- Make sure, as somebody else stated to turn shadows to low, SSAO should be off, Sunshaft (I think is disabled by default on Mac clients?) should be disabled. Vsync set to off. You can lower your View Distance but honestly, with what you have it really shouldn't be that taxing. Also, cap your fps as there's currently issues with overheatings of the graphic cards for both PC and Macs.
http://us.battle.net/wow/en/forum/topic/6490539797 .
While I'm not a big fan of the Mac tech forum of Blizzard's, you can try going through the list from here:
http://us.battle.net/wow/en/forum/topic/1965575695 There's some reset things you can do to the PRAM and something else. You can also try repairing your OS or see what a hardware test says.
2a- I seem to remember reading you tried without addons, or did you? If you didn't try that. Update them or start fresh and redownload them from Cursed or WoWi ensure they're meant for MoP. Not to sound like a robot but please move your Cache, WTF and Interface folders to your desktop and restart the game. Lulz? But, it's possible something could be corrupted but doubting it. This is gonna seem obvious, but make sure your game is up to date. =P
3. Keep in mind, Attip and others. World of Warcrafts' graphic engine is super uber dated. This isn't a game like BF3, Skyrim or the likes where you're more or less required to have a really good machine in order to run the game semi-comfortably. WoW is meant to run semi-alright on some of the worse sludgy computers around and improves from there. The min req. is listed as an i3, it can run comfortably below that and definitely won't have issues well above it.
To put it in perspective how "special" World of Warcraft's engine is, I play on a mid-2007 Intel Core2Duo 2.4 GHz iMac with 3GB of RAM and a sweet ATI Radeon HD 2600 Pro 256 MB (I know, y'all are jealous!) with all settings at Fair to Good, minus things things I listed above and have never seen a single fps issue nor do I dip below 35-40fps, including in raids. This is on Stormreaver so the pop is fairly high, especially in the crowded city in the vale. Granted, I don't usually dare open up Safari in fear of my computer blowing up since MoP was released, but outside that fear everything's fine. This is with 100 or so addons.
4. That said, the performance Attip is having is not 'average' nor is it remotely acceptable. It's great to know a desktop gpu chip will outperform the mobile 680MX chip, but this chip ain't no sludge either and nobody should have any issues running WoW at Ultra with this gpu.
There's still some information that's not been provided by Attip before I'd bash the iMac or tell them to go play via Bootcamp, which technically will run WoW mucho better as well as every other game. But there's no real reasoning his iMac shouldn't be performing better when it comes to WoW.
So, either A- do some (a lot) more problem solving to try resolving the issue. B- Play in Bootcamp or C- Wait for the next OS update and hope it includes the rumored updated drivers. Still some more options to go!