Perhaps I wasn't clear - I meant, if you put current GPUs, like the mobile R9 Radeons in a pre-2012 chasis they would proabably hit the same temperatures as they do in the new, post 2011 iMacs. The "new design" is not the reason they run hot as a lot of people imply (by asking "why do I need to have such a thin iMac"), it's the iMac all-in-one design in general. And this design has benefits and tradeoffs. If I went to some PC gamer forum and started nagging how some large desktop monster has a bad design because I can't put the whole computer on my desk, people would laugh at me.
And when you look at it, it's only the gamers that have issues and are unhappy. Find one post here that complains about heat or GPU that is not gamer related. Like "wow, my iMac gets really hot while I do Photoshop", etc.
So all I'm saying - these iMacs were not designed with gaming in mind, they have awesome GPUs for what they are meant to do - productivity, art, coding, office work, music, design, photography, etc.