Here's a summary of what I have learned from this mini-survey so far:
First, thanks to everyone who has contributed. Keep those ideas and observations coming!
Conclusion 1: For X-Plane, cooling matters more than horsepower. If you were building a PC system, it would be more useful to put that last couple of hundred bucks into cooling than CPU or GPU speed -- because if things are overheating and then have to slow or shut down, or break, it doesn't matter how fast it says they are on the box. (However, some have very fast hardware and no special cooling, and all is well. Me? I'd definitely get liquid cooling.)
Conclusion 2: Macs, both MBPs and iMacs, have more overheating problems than PCs. This is probably obvious given the compactness that Apple favors, but it is also a factor of dust getting into small nooks and crannies. Mac owners running X-Plane should definitely become cleaning fanatics. (I plan on doing just that, and have ordered some P5 screwdrivers and compressed air to handle that -- I will report on my success when they arrive.)
Conclusion 3: The software Macs use to control the fans is too slow to respond to the sudden bursts of CPU, GPU, Power supply, and HDD demands that X-Plane puts on them. I bought a great piece of OSX software called TGPro -- Temperature Gauge Pro -- for $16 and set it to auto boost mode on the default settings. This means that whenever any of the dozens of heat sensors in a Mac rise above 80C, the fans jump up in a hurry. The result is a Mac noticeably cooler to the touch, and no frame rate drop. (Even my other Mac running my programming applications is benefitting from TGPro.) Of course, the fans will wear out sooner, but I figure that is better than something melting down.
I have been using, programming, and making music and film on computers for 33 years. I have never seen anything close to the demands that X_Plane puts on a computer. It's not just that things spike -- they go to max and stay there for a long time. The wonderful IXEG 737 seems to be the most demanding plane of all for me -- maybe because it is scripted, not coded in a lower level language like C or C++. Getting it to perform on a machine that handles it gracefully is a challenge. Well worth it though.