Just got new 4.2 GHz i7 27" retina 27" iMac with the 8Gb video card. X-Plane 11 regularly registers 100 degree C core temperatures with the CPU, even with the fan speed ramped up with a fan controller. I've tried disabling turbo boost, which makes it a bit better, but not much. Performance (fps) is great. Frankly, I'm not prepared to wreck an expensive iMac, and can carry on using XP-10, but it seems a pity. I've cut reflections down to minimum, cut down the objects (buildings, etc.), but this should surely not be necessary.
So, I took another tack. I have found that increasing some sliders in the rendering decreases the frame rate (as expected), but has the side effect, and benefit, of then reducing the CPU temperatures to more acceptable levels. Maybe that is the answer! Just slow things down! Comments please!