We increased our Thermal Target to 110%, which is the maximum PrecisionX allows. We managed to increase GPU clock offset by additional 65MHz. Seeing as how the GPU is already factory overclocked to 1046MHz, this is quite nice. EVGA GTX 660 SC ran at maximum 1189MHz.
Note that the fan was in Auto mode throughout the test. We did speed up the fan to its maximum but that did not help gain any higher on long term stable GPU clock. We overclocked the memory by another 175MHz (700MHz effectively).