We found that there's only a little overhead in the rating Corsair provides - we tried optimising the memory timings lower than the rated 8-8-8-21 at 1,600MHz and found that that at 1.66-1.74V: 7-7-7-18 would not boot, 8-7-7-18 and 8-7-8-18 would boot, but fail to run most of the benchmarks, but 8-7-8-20 would run them all, albeit slower (sometimes considerably) than at 8-8-8-21. Increasing the voltage had no effect and in fact we found that over 1.74V had a negative effect with a loss of stability at known settings.
Trying for maximum clock speed also had trouble, and we were limited to 1,708MHz at 9-9-9-24-2T at 1.74V (again we hit the voltage "limit"), with a base clock of 142MHz, 1.4V on the Uncore area (this was the lowest stable setting, but up to 1.45V made no difference and above this turned the effect negative again) and 1.35V on the CPU. We had to raise the base clock, even despite our 965 Extreme Edition Core i7 CPU having extra memory clocks overhead, however it would have run the memory controller at speeds we know weren't possible for our CPU.
Strangely, we have achieved 1,800MHz from this exact same memory, motherboard and BIOS combination before, but our luck seems to have run out this time around because nothing we did (BIOS settings, extra cooling etc) would yield greater results. Memory speed and overclock is highly dependent on the CPU (like it was once the motherboard) with different batch numbers affording large differences in potential - if you are buying high performance memory, it could simply be limited by the processor now.