DO you have the right version of the AMD driver installed? I installed from the AMD site somewhere in Oktober the last version of the blockchain driver, and after starting up I had a very bad hashrate (20 MH, while I should have 24 for a standard card). There was a bug in the AMD driver, so they have now released a new version of the Driver. Try to uninstall the AMD driver with DDU and reinstall the latest blockchain AMD driver.
8 seems indeed a right number or you could go to 13 with the Asrock mining motherboard. 20 seems to high to me and I would only do it if you are sure you do not have to move it.
I think you should always be careful on advice from however it's coming. Always do your research and if many people say the same and the story and technology is good than you should consider buying it.
I'm Dual mining with Claymore mining. I'm mining Ethereum and Decred. I found that siacoin mining goes to slow and Decred gives a nice extra gain, without using extra energy.
As the pice of both coins has increased significantly in the last month it becomes a lot more profitable to mine them