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
wait, there is a DP switch for cudaminer? what does it actually do? (aside from apparently lowering your performance)