Try lowering the intensity to 13, the 280x's work best there. Also, change the thread concurrency To 8192 then play with the clocks. Should do the trick!
already have done that on my other rig, i got 11 of these cards with 7 set up so far, one rig is running the config i posted above, another rig has 3 cards running this
-I 13 -g 2 -w 256 --thread-concurrency 8192 --gpu-powertune 20
only achieving 550khash... i've messed around with the clock in 5mhz incriments up and down like you would not believe... no joy