I have two MSI's. One is at 1315 MHZ and the other 1200. Memory is on deffault. I get now 1500-1600 kHash per card. The difference because of clock speeds is minimal; around 40 khash. My config is the same as yours... Not sure why you get only 1280. I'm using separate instances for my cards if that helps.
I will test it out later with just one instance on a card... not sure if I can get 6 instances running, but maybe I can double them up.
Haven't had to run multiple instances of CCminer on anything lately...
Thank you for the input though, I will test and report back if it helps.