Thats where I am too right now. Except how are you getting 700 on your 280x?! I'm getting right around 500, just like the 7970's. Different settings than the 7970's?
Unfortunately, the end is coming soon for those older cards and 14.04 I think. Plus the RX 470 / 480 uses so much less power. Something needs to improve soon with 16.04 and the amdgpu-pro drivers. Have to install Windows just to tweak those cards at all!
Amazing the amount of work that's gone into SGM 5.5 over the last few months. Compiles and runs in Linux like a charm. "What do you mean you can't find OpenCL, it's right **** there!" That all went away. Documentation went from like zero to pretty darn good. All the noob tutorials are now officially defunct, which is probably just as good.
Would personally recommend Xubuntu in version 14.04 if you have a card that'll work with fglrx; my older gen cards are killing it. 290x hashing 700-740 with no mods, 7950 500-520. 7850's getting 340, well in excess of what they get under windows/Claymore. SGM just screams in 14.04. It'll run from console, SSH, mosh, you name it. All on some, like $30 army-surplus motherboard I got off ebay. I've tried about umpteen distros over the last few weeks. My advice, for xmr anyway, is keep it simple. Go with some ubuntu flavor (I'd do xubuntu or mint, except for cinnamon), don't go nuts on your config file. Every card I've tried so far likes 2 threads, and it'll pretty much always run with a raw intensity of 512. Except for my 280, which I agree is kind of a conundrum.
Can't say I'm as thrilled with 16.04, which is kind of a pig. Somewhat interested to see where radeon goes with this. Bet they ditch X altogether. Bet everyone does, except ubuntu. I won't miss it.