I did a bit of testing a year or so ago and found that most GPU's used around 75% of the TDP when crunching here. Although I had read in a review that the GF600's were different I was still slightly surprised to see a GTX660Ti purporting (Gigabyte OC Guru and GPUZ) to use 93% of it's 150W reference TDP when crunching. It's at 57°C so temperature wouldn't be an issue. The 1.175V might on the other hand be worth looking at.
However, it auto boosts to 1188.9MHz which is 21.3% over the 980MHz reference boost. When you factor that in it's almost spot on that 75% guide-mark at 76%.
I'm not really surprised at that as modest OC's usually improve performance fairly linearly compared to the Wattage of the GPU.

How much of the TDP are your GPU's using?

Just for reference, my GTX660Ti on W7x64 is 30% faster than my GTX470 (FOC) on XP and in terms of efficiency it's 50% more efficient in terms of throughput/Watt.
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GTX660Ti + Win7 64, using 69% of TDP at 1254 MHz, 1.175 V. Apparently it's got about a 15% higher TDP than stock cards, though.

MrS
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Scanning for our furry friends since Jan 2002

MrS, it seems your system gets through tasks around 3.5% faster than my similar rig. 4.2GHz/1188 (now varies from 1175 to 1201). Differences explained by clocks. Was 290W at the wall when crunching 6 CPU tasks and 1 GPU task.

I added a 470 @656MHz, and the 660Ti is 33.9% faster.
I didn't see any performance drop when moving from PCIE3.0 x16, to what I presume is PCIE3.0 x8 (for the 660Ti) and PCIE2.0 x8 for the 470. 96% and 97% GPU utilization. 43% and 27% memory controller loads.
Idle is 90W at the socket, and crunching (6 CPU + 2 GPU) is 450W.
Corsair HX750 PSU (@91% efficiency).

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Wow, that are some consistent numbers! My idle power draw is 60 W, load is ~215 W with no BOINC project running besides GPU-Grid.

MrS
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Scanning for our furry friends since Jan 2002