Anyone has idea why 280X with its huge memory bandwidth has such poor ethash performance?
Try mining with a small dag (i.e. Ubiq, Expanse). Part of the issue is the memory controller isn't as efficient as GCN3 and later, but most of the issue is the large DAG size combined with 4K pages causing TLB misses.
I haven't tried it, but the 2M page support should work with Linux kernel 4.15, which would make a big difference mining ETH.