first off, assuming you're running on your sig's DFI LP x48, that board is not known for great memory compatability.
it may be paranoid of me to jump straight to a mobo problem, but in my experience that's where most problems come from, especially with respect to RAM speeds.
It's hard enough to find a mobo that will reliable support 4gb DDR3 at high speeds, let alone 8 gb. My guess is it's a RAM thing, as this stepping has been really good to me in terms of clocks. The fact that y0u can get higher at 4gb than 8gb supports this. also, 1.6v isn't that crazy for nb volts.
try loosening your timings and strapping a lower divider and see where that gets you.
if that doesnt do much, than i'm totally wrong and should stop running my trap.