I have a RX580, which is pretty much the same thing, and I have been tweaking for a few days and I have gotten it hashing at over 10 mh/s.
M
The secret? try GPU threads set at 4, aka -g 4   I have a AMD Ryzen 7 and 32GB ram, oh and i overclocked the GPU core to 1300MHz and the memory clock is not changed (2100).

here is the rest of my config:

"profiles": [],
  "failover-only": true,
  "algorithm": "x16r",
  "device": "all",
  "kernelfile": "x16",
  "xintensity": "256",
  "gpu-threads": "4",
  "temp-cutoff": "95",
  "temp-overheat": "85",
  "temp-target": "80",
  "gpu-memdiff": "0",
  "shares": "0",
  "kernel-path": "/usr/local/i586-mingw32msvc/bin",
  "api-mcast-port": "4028",
  "api-port": "4028",
  "expiry": "28",
  "failover-switch-delay": "60",
  "gpu-dyninterval": "7",
  "gpu-platform": "-1",
  "hamsi-expand-big": "4",
  "keccak-unroll": "0",
  "log": "5",
  "no-pool-disable": true,
  "no-client-reconnect": true,
  "queue": "1",
  "scan-time": "7",
  "tcp-keepalive": "30",
  "temp-hysteresis": "3",
  "watchpool-refresh": "30"