I have 4 x R9 290's and I tried with various options in the sgminer using Windows 8.1, best I saw was about 8mh/s for all 4 cards. A 290 and a 390 are basically the same - with the 390 having more and faster vram. I do have the old 15.12 drivers installed, maybe that was a problem?
I saw a list somewhere suggesting that a 390 should do 10mh/s but I'm sorry I don't know how to set the miner to achieve that. I tried Intensity at 20, 2 threads per GPU, --gpu-platform 0/1/2/-1 and a few other things I can't remember right now but like I said, only about 2mh/s per GPU.
I have an Nvidia rig (2 x 1070 & 3 x 1060) doing 80mh/s.
So I am also interested if anyone has any suggestions about getting AMD hawaii cards to hash SIGT faster?