Just quoting Optiminer's GitHub:
Windows
Full speed can only be achieved under Catalyst 15.12 drivers! See below for how to install the older driver version.
RX4*0 cards are not supported by Catalyst 15.12, I strongly recommend to use linux for mining on them!
I use Windows, therefore Optiminer is not a very good option, according to them!
i use 4 day Optiminer on Tonga. win 10 with 15.12 driver 185-195 S/s with "standard" win 10 driver (16.6) 207 S/s