I don't personally have 280X, I used to mine with a bunch of GTX 970, 750 Ti and 960. I got 1,3 GH/s for each 970, so getting 2,2 GH/s per 280X isn't surprising I think. That's data that I grabbed on the web, so it might be false. In all the cases, you'll always have at least 1,2 GH/s.
lol what Asus Fury X - 2.5 gh/s, 7970 - 1.3 gh/s, what version of 280x can do 2.2? or what ulta-supa-bupa miner need to use, tell us pls
thanks