I have GTX TITAN, but performance looks similar (maybe not used DP):
with turned off DP: 485khash/s @ 770Mhz with -l T14x32
with turned on DP: 480khash/s @ 770Mhz with -l T14x32
btw. i don'n know, why clocks are such low with the latest version of cudaminer 18-12-2013, while gaming frequencies are 1045Mhz
If there is some bios editor which can turn off stupid turbo...it would be great :-)
There is KeplerBiosTweaker version 1.26.
However if you are running on low frequency it is most likely due to high temps/high power usage (assuming it works like the 780ti).
Try to increase the fan speed first (the card is by default optimized on silence....)