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.
Update on what I have found...
Running one instance of ccminer50 v1.0 per card for my first two cards works like a charm. First card is running at ~1780khash/s, and the second ~1670khash/s.
Firing up a third instance of ccminer, dropped both cards to ~1400khash/s per card, and if I killed that third instance off they would ramp back up to full power.
I had also discovered that having left jackpot-qt running (for the stake coins) was killing the hashing speed if I left it running on the miner, so I moved that over to my main machine off the mining rig. Not really sure why that was such a problem for the hash rates, but that was the first thing I discovered.
I tested running up to 6 instances of ccminer, and that ended up pegging my CPU at 100% anyhow, so not really conclusive other than I don't have a CPU in the miner good enough for that (1150 socket 2.7GHz Dual-core Pentium Haswell, 8GB RAM).
So for now I am having two cards mine DRK, and have the other 4 sill solo-mining JPC for now... and that seems to not be slowing the DRK mining cards down at all having one other instance open running the other 4 cards.
Anyone running 5-6 cards mining x11 right now? or are most of you running just 2-3 cards?
Thanks for everyone's help and suggestions.