I'm not sure what the default X2 CPU NB volts are TBH, as they should all differ based on CPU VCore. IIRC, the VCore and IMC are fed the same voltage on K8's and the BIOS won't allow you to choose vCPU NB with an X2 separately (IME) whereas with Phenom the power states are separate and both can be fed different voltages. Can you choose vCPU-NB manually with a K8 Rammsteiner?

If you see it in the BIOS when X2 is installed then it will be a setting for it. Only options you see changeable when you install a CPU are those that support it. From my memory, it wasn't there with a X2 BE-2350 on the ASUS or the DFI 790Fx.