Under linux OS like smos and hiveos I get 27-28 Mh for 280x. There is only one tweak in the config that I've made under Ubuntu and that is the set parameters for the virtual memory. The only drawback is the power - around 180-200W vs. 90W for 1060 which get 23 Mh.
radeon 280X had 288MH/s default memory bandwidth, even RX 580s could not surpass that (256MH/s on default clocks) while 7950 had only 200MH/s (which still means ethash should probably be close to 20MH/s, and not only 11). But probably
old architecture and lack of optimizations affect it, too