In the cudamining.cc release page, is ther a version released for x15?
I know that nvminer has it but I wanted to compare it to ccminer. For some reason on one of my rigs I can't get the nvminer over 45% gpu load with x15. If I use ccminer on it it works fine (jackpot).
Try djm34's version of ccminer just to make sure I didn't break anything when making nvMiner. Let me know if you get anything different with his version.
BTW, what hash rate are you getting on x15?
Carlo