Ok, so I've finally made my first run.
Might be it was higher than 38 when istarted it first after all.
1. Switch setup
Room Temp (estimate): 22°C
Charger: Unofficial
Docked: N
Android Performance Setting: Max Performance
2. Any thermal modification?
If Y:
Thermal paste: MX-4
Copper Shim removed? Y
Cleaned under RAM shield? N
Thermal pad over RAM area? Y
Thermal pads over heat pipe? Y
Anything else of note? N
3. CPU Prime Stress test
Starting Temp: 37,2°C
After 2 Minutes: 38,6°C
After 5 Minutes:40,1°C
After 10 Minutes:41,4°C
After 15 Minutes:42,1°C
After 20 Minutes:42,6°C
After 25 Minutes: 43°C
After 30 Minutes: 43,1°C
After 30 Minutes, it seemed like things wouldn't actually get any higher so i stopped.
Doing a quick reset back into hekate revealed something weird, namely that it measured a different temperature of 46,8°C... I'll do another run once it's cooled down just to see if whatever it displays ends up being in the same ballpark as the first time. Even if it's not the true temperature, it'd still be comparable then.
Second test, started at lower temperature, but while it seemed like the initial heat up would quickly close the gap, temperatures rose a lot slower this second time around. So maybe this was all for nothing
CPU Prime Stress test
Starting Temp: 35.5°C (This seems to be about the idle temp from just waiting in the homescreen for a while)
After 2 Minutes: 38,3
After 5 Minutes: 39,5
After 10 Minutes: 39,9
After 15 Minutes: 40,5
After 20 Minutes: 40,9
After 25 Minutes: 41,3
After 30 Minutes: 41,4
Hekate again higher at 43,7