Is my iMac overheating

Is my iMac overheating ...

No. Not even close.

If a Mac's internal temperatures become excessive and attempts to limit CPU performance were to fail, its SMC will shut it down automatically without any user intervention.

Apple builds portable Macs that they do not characterize as "laptops". Their marketing term is "notebooks". If you want to use a portable Mac on your lap, that's up to you. Excerpted from Mac notebooks: Operating temperature - Apple Support:

The bottom of your Mac notebook may become very warm during normal use. If your notebook is on your lap and gets uncomfortably warm, move it to a stable work surface that allows for good ventilation.

If you want your Mac to last do not use clever utilities designed to circumvent a Mac's cooling system. Uninstall "Macs Fan Control". Arbitrarily increasing exhaust fan speeds in an unjustified attempt to reduce internal temperatures will only draw in needless airborne contaminants, accelerating the eventual need to remove them, and potentially increasing eventual hardware failure.

Have you installed the Boot Camp drivers? This may help with power management - the temp. are way too high as for MacOS I would use a custom profile in the Fan Control app to ramp up the RPMs in order to keep the CPU at least under 90C.

Coming from the PC world, running a CPU at stock even at 80C is just really bad, sure the CPU won't fry as it will throttle but the surrounding components will get stressed by the extra heat thus reducing their efficiency and finally their life.

My 3770K (PC) never goes over 70C @ 3.9Ghz and that with no fancy cooling since we're talking about stock freq.

If you're performing a stress test, obviously the temps will be high. That's what a stress test is designed to do - push to the limit. Nothing is failing, so its fine.

Compared to cheap Windows boxes (not all PC), Macs generally use higher quality computer components to withstand higher stresses.

If you're not seeing red temperature warnings, your Mac is not OVERheating. Installing third party fan controllers is not just notable required, but sometimes interferes with the built in controls. Macs are very good at taking care of themselves.

It wasn't my intention to turn this into a Mac vs. Pc thread - even if what you say is true - at 100C my Macbook Pro Retina - whilst compiling something like Servo - makes the machine unusable on my lap (too hot), therefore I'd rather ramp up the dBs and be able to use my laptop.

Regarding the part where the software interferes with the built-in controls can you link some factual evidence and how that is detrimental?

Under high CPU load these kind of temperatures are quite alright. The fans are kicking in at max, I suppose?