Set the fanspeed to some fixed value, the thermal limit to some fixed value and the power limit as high as it goes. The run a stress test and record the package power after some determined amount of time (and check it is actually at the thermal limit and not limited by something else, if it isn’t reduce the thermal limit and retry).
Imo that’s a pretty solid way to test relative cooling performance.
I did do this kind of testing between stock and ptm earlier in this thread over a range of temp limits and fan speeds (I also did the testing with lm but the results were within margin of error of ptm so I didn’t bother to update the charts).
Edit: Almost forgot, make sure you do it on the same surface, that makes a huge difference. Propping the back up just a little drops temperature by quite a bit at max fan so that’s a variable you’ll want to control.