74c is very far from overheating.

Just because the thermal pad doesnt show the heat gathering like the open space between the pcb and backplate doesnt mean it isnt still there. There is no airflow in that case, which allows heat to gather.

If they closed the case, the airflow would remove heat.

No, there isnt a need for any thermal pads on this card too.

As far as I understand, thermal image doesnt show through glass or solid surfaces. Am I misunderstanding that somehow?

So, there couldnt have been positive pressure during the test.

I dont think you understand what i am getting at. You didnt have positive pressure during the test with the door open. Even you admitted that. Without positive pressure, the airflow is going to be reduce over the back of the GPU, as the air is judt escaping out of the side panel and not going over the card.

Try placing a fan blowing in the general direction of the card so that it gets airflow across the card, just like you would aith the side door closed, or close the thermal reader in the case and let it read, so you can see how much difference there is.

Also, I have 24 fans in my case. 12 in each chamber. That doesnt change the airflow fact when the case is open.

Again, the temps a not bad. I think you may be see something that isnt there. The card isnt getting near what the ACX coolers got to, which is what caused the concern.

The thermal pad on the ACX cooler is also going to spread the heat a lot, so it will read lower as it is dispersed over a larger surface. If you want to put a thermal pad in there, order one and throw it on there. It wont cost more than a few dollars. The temps you are showing are of no concern unless you make them a concern.

As for the build up, putting a pad in the back would only mask that, and you wouldnt be abls to see the true temp, so why would you do that?