I don't know if it works with MOD bios. I don't mess around with that - maybe someone else knows. It works just fine with RX480's. Here's a picture of Claymore 11.1 linux on Ubuntu 16.04 LTS using 6 RX480's:
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did you ever tried Optiminer on Ubuntu 16.04? People get 280-290 H/s on RX480 AFTER fees, not before (what you are posting).
Small difference, but still.
I get 305 per RX480 with optiminer
I have 18 RX480, do the maths, its a huge difference.
what driver are you using? I tried and only got 265. On the 7950 gave me 6 (yes six)