Is the hard drive screwed on, or when you remove the rubber cover and SATA connector it come right out?
Can you check your fan speed RPM? With istat or something like that? I think the max speed is 6000RPM, so let't hope that upgrading the HDD won't give the same problem as the 2011's iMac
Maybe it has something to do with the Radeon 6630m model? I ordered the Core i7/6630m model, I'll check when I get it and report anything different I see.
Is there any hard drive sensor cable this time around? Less small cable to remove/put back = less chance of breaking things! :)