First off, I can't thank c-attack enough. Without their thoughtful and detailed reply, I'm not sure I would have undertaken all this testing.
tl;dr H100i RGB Platinum SE LEDs add 6.3C to the coolant temp. IMHO, this is probably due to the proximity of the temp sensor and the LEDs, and not representative of actual increase in coolant temp.
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Well, I'm back after several hours of testing. The results have been very interesting.
I went through several stages of troubleshooting and made detailed notes at each stage. In summary, I went from 41.5C to 21.4 on the coolant temp by doing the following things:
Fixing AIO fans and pump to their maximum speeds Removed top glass and dust filter Turned off all RGB in the system
Even though I have no intention of running my PC like this, I was able to determine that there was likely nothing fundamentally wrong with the AIO, particularly the coolant temperature sensor.
The lowest coolant temp I achieved tonight was 21.4, and the highest was 38.9C. The breakdown is as follows:
Difference between 600 RPM and MAX on the fans: 7.2C Difference between "Balanced" and "Extreme" on the pump: 5.1C H100i's RGB: 6.3C
Yep, the RGB on the AIO added a very fast 6.3C to the coolant temperature, and the temperature dropped just just as rapidly when the RGB was again switched off. The rest of the system's RGB appears to have no significant effect on temperatures.
Given the speed with which the coolant temp rose and fell, I suspect that the LED heat didn't actually get transferred to the coolant, but that the coolant temp sensor picks up heat from the LEDs, leading to this 6.3C discrepancy.
With the case reassembled, I turned off all the RGB and set the AIO fans to 1,300 RPM. (This seemed to be the sweet spot to my ears.)
Coolant: 28.8C, CPU Core 32C
I cranked the fans up to maximum and re-ran my stress tests. (Based on my data, I'll make my own fan curves. For now, I think it makes sense to stress test with max fans.)
Coolant: 39.4C, CPU Core 71C
IMHO, the 6.3C added by the AIO RGB is very significant and should be noted and/or fixed and/or accounted for in built-in fan curves.