The discharge or out line is converter discharge oil. It will tell you the temp of the converter discharge but NOT what the trans is seeing.
As for the temp bung in the pan, that can be what we call "stagnent" oil meaning, it is really not always in the circuit. By measuring at the return cooler line shows EXACTLY what the lube return circuit temp is and what the lubrication temp is.
Hope you follow this and if not, let me know and we will continue this discussion.
But, I hope this makes sense to all.
BTW: Our pans do have an 1/8th pipe plug in then for the drain, but to cheat and install easily you could put it there.
Bruce
WE4
Performance Transmission Services