Have tried Claymore's zcash miner 12.4 under the same Ubuntu 16.04. It shows me ~800 H/s per 3 RX480 cards while sgminer-gm showed only 570 H/s. How is that possible?! Is Claymore's miner lying?
I am getting about 305 H/s per RX480 in Windows with Claymore's Zcash miner. I get the same result on average from the Zcash pool where I mine.
I have not tried sgminer for Zcash.