You will also have to enable C1E if you want to also lower the vcore. My board disables it though over a certain OC.
Also I am stress testing with C1E enabled currently but how do I actually test for the underclocking stability?
Currently im using:
378x9 = 3.4ghz@ 1.30v
1.29 NB voltage, 2.11v DIMM
35*C idle, 55*C load and 70*C load with only the H50 fans barely spinning, zero P95 errors so far (I have changed the thermal config a few times for one hour each to see what the temp would do).
So far this board has been an absolute dream