@GLSRacer thanks for your sharing your experiences, much appreciated! If you are running as a server I would not expect much GPU load to be seen by you and thus no issues either.
On the temp issue, I have just breadboarded the thing and have applied the latest in precision temperature measurement equipment (my right index and middle fingers) and spot cooling solutions (blowing on stuff). I have come to the conclusion that the Nuvoton sensor on the A300 labeled "CPU" must be reading SOC VRM temps, or somewhere really close to that area of the board.
First I removed the VRM heatsink and fired up a CPU stresstest while observing the sensor readings. CPU internal sensor went up as expected and Nuvoton CPU sensor stayed almost flat as expected. The 3 MOSFETS that supply CPU core voltage got very warm to the touch but not to the point of being painfully hot.
I then used furmark to perform the same check for GPU. While GPU internal sensor did climb slowly, the Nuvoton "CPU" sensor hit 50°C almost immediately and only went up from there, I tested up to 72°C reading. The SOC VRM MOSFETS got increasingly hot to the touch to the point where leaving the fingers on for a couple seconds got painfully hot. Blowing on the SOC VRM area at this point did cause the sensor reading to drop almost 10°C. Blowing anywhere else onto the board (CPU VRM, around the socket, around the RAM etc.) would not cause a drop in temperature.
Lastly I remounted the VRM heatsink and ran Furmark again. The same sensor that easily hit 72°C previously appeared to settle into steady state at around 62°C, further supporting that the temp reading is VRM related.
So yeah, still not quite sure what to make of it. I may have had a bad mount with the VRM heatsink. Or i may have a bad batch of SOC VRM MOSFETS. Or this is just normal, even though I don't think 90°C on VRMs is great. I'll have to put it back into the case to see if it has changed, the delta between GPU temp and "CPU" sensor temp was only 20°C without case after remounting the VRM cooler, that is 10°C less delta than before.
Generally thermals outside of the case are soooo good by the way.
I would still love to hear anyone elses observations (also on the SOC voltage thing, I will do more tests there as well).