The thing I noticed is the fact that my maxed-out 21.5 doesn't get hotter than 75C/70C for CPU/GPU respectively under full load (OCCT) while being pretty silent. Keeping in mind, that 650m in 2012 iMac is pretty overclocked (as rMBP's one), it's TDP probably goes around 55W instead of 45W stock. 670MX has 75W TDP. Assuming the fact that cooling system is pretty much an overpower for its components, the question is - why there's no 670MX BTO? What do you guys think? And btw, who would like to get an iMac with that kind of GPU?