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.
Are You talkkng about R9 280x 3GB ? I have few of them and my max is 12.6 MH/s
usung 5.3c + Windows 10 + driver 20.4.x
https://prnt.sc/vu5tjqDo You think that on linux is chance to get more ?