No matter the ambient temperature (within reason), the GPU will reach its maximum temperature during sustained 100% usage. The rate at which it reaches it will differ, and the rate at which the fans must spin to keep it from overheating will also differ, but it will ultimately reach the same temperature regardless of air-con or not.
Sorry mate, but the above is simply and categorically not correct. I am afraid you are mistaken.
I don't say this as a matter of opinion, but as a statement of scientific fact.
In a freezing cold room at 0C, if you get a 80C GPU reading after say 20 minutes of running, then that same GPU will read precisely 100C if you do the same test in a 20C room. Or 101C is the room temp is 21C. It's absolutely directly correlated.
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If you are interested to understand why, here's why:
Temperature is a function of heat and in a steady state, the temperature of a GPU (or indeed anything else in the universe) is such that the rate of heat input matches perfectly the rate of heat output. This is basic physics.
As you input energy into the GPU, its temperature will rise until such point that the rate of energy coming out as heat is the same as the rate of energy going in. (Rate of energy = power = watts, by the way.) It is then in a state of equilibrium.
But the rate of heat output is deteremined by the temperature of the object
in relation to its surroundings. Heatsinks for example are measured in their ability to dissipate however many watts, per degree C... above their surroundings. The bigger the temperature
difference the faster the rate that heat is emitted.
The difference between the GPU temperature and the surrounding room temperature is what determines the rate of heat dissipation, and therefore determines the temperature at which equilibrium is reached.
Therefore, in a steady state, the GPU temperature is precisely and directly determined by the room temperature.
Try it if you don't believe me! Take you iMac outside and run the same tests and see what results you get! They will not be the same as inside, if it's colder outside (or warmer!)