You can increase the speed with higher intensity. Why don't you try it out. What numbers are you getting?
--xintensity 224 (default)
--xintensity 256
--xintensity 288
--xintensity 320
I am getting 64-65 MH per card on xintensity 160/256/512.
It seems as the miner submits more accepted shares on xintensity 160 vs 256.
Thoughts?