The earlier versions showed a bit of a decrease of the performance in Ethereum while using the dual mining mode, but the latest improvements have significantly improved the situation, including the results for slower AMD GPUs. With the latest version 3+ you can get slightly more Ethereum mining hashrate than with the regular ethminer while at the same time you are also mining some Decred. The higher-end the AMD GPU you are using, the higher performance gain you can expect – from about 1 MHS on 79×0/280x cards to a few on 290/390 series of cards plus the extra Decred mining hashrate. Interestingly enough not mining in dual mode, but only Ethereum is not showing difference in the hashrate now on a Radeon R9 280X for example.
So if you still have not tried the Claymore Dual Ethereum and Decred Miner for AMD GPUs (it does not work on Nvidia video cards), you should give it a try. Do note that although this miner is free-to-use, there is a small developer fee of 1% for Ethereum-only mining mode and 2% for Ethereum plus Decred mining mode, so every hour the miner mines Ethereum for 36 or 72 seconds for developer. Also, if you have been mining only Ethereum for the moment and you switch do dual mining mode adding Decred you should know that the power usage and the heat generated by the GPU will increase. So make sure your AMD GPU mining rig can handle the extra power usage and heat generated when dual mining, because Ethereum is less power hungry and more memory than GPU demanding algorithm while the situation with Decred is the right opposite and thus both can be mined together with pretty good results. Another important thing to note is that the miner is only released as a Windows binary, no source code or binaries for other operating systems are currently available.