I use on my RX 480 8G:

{ "id" : 0, "intensity" :56, "worksize" : 16, "threads" : 2, "target_temperature" : 60},
{ "id" : 1, "intensity" : 56, "worksize" : 16, "threads" : 2, "target_temperature" : 58},
{ "id" : 2, "intensity" : 56, "worksize" : 16, "threads" : 2, "target_temperature" : 55}

and it gives me more 1000H/s