you can use the nano mod of that guy in this forum, he mine with 75watt and 22 mega i remember, pretty good, i still need to see that super efficiency of other amd, it's strange that they cna match nvidia, like the 390 for example
If you under volt the R9 390 to less than 1000 mV, you can get around 6 J/MH, that is better than 970.