I got only 280X, but try to play with these settings:
"intensity" : "14",
"worksize" : "64",
"kernel" : "Lyra2RE",
"lookup-gap" : "2",
"thread-concurrency" : "8192",
"gpu-threads" : "2",
"gpu-engine" : "1024",
"gpu-fan" : "10-85",
"gpu-memclock" : "1500",
"gpu-powertune" : "0",
"temp-cutoff" : "75",
"temp-overheat" : "69",
"temp-target" : "65",
"api-listen" : true,
"api-mcast-port" : "4028",
"api-port" : "4028",
"auto-fan" : true,
"expiry" : "20",
"failover-switch-delay" : "60",
"gpu-dyninterval" : "7",
"gpu-platform" : "0",
"log" : "5",
"no-pool-disable" : true,
"queue" : "1",
"scan-time" : "60",
"temp-hysteresis" : "3",
"shares" : "0",
"kernel-path" : "/usr/local/bin"
Are those your configs for the 280x?
What is your hashrate with the 280x?