The thinness is at the edges. There's still a lot of volume in the center. A Sandy Bridge 27" iMac has 3 fans actually -- one for the CPU, one for the GPU, and one around the HDD.
The Sandy Bridge 27" runs silent under hard gaming. I'd expect the 2012 to do the same, considering the CPU's TDP is 18W lower.
The GTX 680M should be a 60-100% framerate bump in games over the 6970M in the mid-2011. The difference in CPU performance will mostly be around 10-20% since clock speed didn't change, but because Ivy Bridge offers some bennies over Sandy Bridge.