So I've gone and dropped a 1090T BE on an 890GX and covered it with a Noctua U12P-SE2. 1x120mm pushing air onto the two noctua 120mm's sandwiching the heatsink and extracted by 1x140mm and 1x120mm.
Naturally I went straight for Crysis and some Premiere Pro encoding, with HWMonitor and taskman in the background. With all 6 cores at full load, my temps began to slowly creep up, and up, and up.... to 23 degrees celsius.
The ambient temp in the room is about 12C. HWMonitor says the 1090T is idling at 13-15C. How is this possible? Surely it must be an error in sensor calibration? I was expecting at least 40C+. In the same room, the P9600 in my Vaio is idling at 40C.
I would say this is quite possible particularly if you have low ambient temps and a good cooling solution in place like you do. My Athlon II X4 idles around 21C and that's with the stock cooler. If you have good case airflow and ventilation and an aftermarket cooler then this is achievable if you live in a cooler area. I wouldn't say its anything to worry about.