I'm using the Corsair XMS3 1600MHZ CL7 (ver. 2.2) too, and the actual Vdimm drops on my MB (DFI UT X58) too. 1.665v BIOS-set ends up as 1.64v BIOS-report, 1.64v in SG (DFI's monitoring SW), and 1.65v in Everest. It has been running stable with this Vdimm on it's rated speed, but raising the tRCD makes it more stable, including on this
HyperPi32M run @1680MHZ 7-8-7-21 1.65v and CPU @4.41GHZ(210x21) 1.32v.
I have been playing with this kit, and it seams to run much more stable with tRCD one notch higher without loosing any bench-number, but I could never get it stable with 1.62v at these speeds.