I also have Sapphire 280X Dual-X OC (11221-00-20G)s and am happy with 750KH/s I'm getting out of them. 710KH/s was the limit on how much I was able to get out of them out of the box.

First thing I did was download then flash a recompiled and optimized BIOS with 1065MHz core as anything higher than 1070 reduced the hashrate.
Then I downloaded a recompiled and optimized scrypt130511.cl plus scrypt130511Tahitiglg2tc8192w256l8.bin for SGMiner.