http://ethosdistro.com/kb/#fglrx-amdgpu

Increasing Hashrate on Tahiti/Tonga/Fiji GPUs with AMDGPU

As of ethOS 1.3.3, you can increase the hashrate of older GPUs (Tahiti/Tonga/Fiji Series) by switching the driver from fglrx to amdgpu.

List of Tahiti GPUs : https://videocardz.net/gpu/amd-tahiti/
List of Tonga GPus : https://videocardz.net/gpu/amd-tonga/
List of Fiji GPUs : https://videocardz.net/gpu/amd-fiji/

NOTE! If you do not have any of the above GPUs, DO NOT PROCEED with the below guide. Please also note that most hashrate improvements are limited to ethash mining.

Follow the below instructions to set your driver to amdgpu.

In your config, set legacydriver [worker] amdgpu / globallegacydriver amdgpu
Then, set miner [worker] claymore-legacy or globalminer claymore-legacy
Run: putconf && sleep 30 && r

After your rig reboots, your Tahiti, Tonga, and Fiji GPUs will have an increased hashrate.