I use Furmark to check the issue of overheating, usually if the thermal paste is bad the temperatures reach 90 ° quickly running Furmark.
I don't think bad thermal paste would cause it to reach 90C. Most of the time when it reaches 90C its usually the fans failing. Usually the bearing starts to make a little noise and then it runs slower and slower. Then eventually when the GPU is cold it fails to spin and eventually it starts rattling bad and that's when the temps might spike.
However the factory paste usually never goes bad enough to cause the GPU to overheat like that especially for the RX gpus which aren't that old. The reason why people do a thermal paste job is because there are better pastes out there and they want to upgrade from the original. But for mining in general the factory paste is good enough.
