i have 6 nvidia 750 ti, and im getting about 15 MH, anything i can add to tweak the hashrate?

two on one rig

ccminer.exe -a x11 -t 2 -o stratum+tcp://mine.fibre.xpool.ca:5555 -u FA15d2Es5t9NGSM3EfPCzumr6EVXBqV38L


4 on the other

ccminer.exe -a x11 -t 4 -o stratum+tcp://mine.fibre.xpool.ca:5555 -u FA15d2Es5t9NGSM3EfPCzumr6EVXBqV38L