Are you running the 280x with g1 or g2?
g1.. can't get anywhere with g2
Run this with -g 2, should be getting 362 Kh/s with a 280x:
"intensity" : "13",
"worksize" : "256",
"lookup-gap" : "2",
"thread-concurrency" : "8192",
"gpu-engine" : "1000",
"gpu-fan" : "20-100",
"auto-fan" : true,
"gpu-memclock" : "1500",
"gpu-powertune" : "50",
"temp-cutoff" : "95",
"temp-overheat" : "90",
"temp-target" : "70",
"expiry" : "30",
"log" : "5",
"queue" : "0",
"scan-time" : "1",
"scrypt-vert" : true
}