I think that even with the proper setup as JohnG described, taping a thermocouple on would affect the temperatures negatively as well. It would likely directly block the airflow on the area of the component hidden under the tape like a miniature tent (or like wearing a wind-breaker outside in a breeze), degrading cooling ability.
This way of testing also assumes that the part of the tape that's flush on the component (rather than the 'tent' portion) has perfect thermal conductivity and doesn't insulate the part. There is a lot of variation between heatsink thermal pastes' qualities, for example, that makes a difference with cooling hot parts.
Maybe I'm wrong.
That said, I don't know a great way to get a true reading on the temp without a professional setup that avoids these pitfalls (calibrated thermal camera built into the side/top of a custom case?).
At a different level, important aspects to analyze are the
airflow and
static pressure specs of the stock fan vs. replacement fan (Edit: Assuming case completely closed, of course, since any additional gaps between panels would affect airflow).
I installed the Noctua NF-A6x25 FLX and it has two working fan settings. I used the lowest one and it works great!
I very much share your desire for low noise! However, we must still take care to compare the noise-lowering solution to the original design specs for cooling ability.
https://noctua.at/en/nf-a6x25-flx/specificationUsing the medium or low Noctua fan speeds using one of the adapters, the airflow and static pressure might be too low compared to the stock fan.
My understanding is that in tight spaces with lots of air resistance, static pressure can be particularly important.
@Blue
Can you share a pic of the stock fan to see its model number?