The shadow of mordor comparison is additionally flawed because you are comparing the 285 with only 2GB of VRAM. SOM requires at least 3 GB of VRAM for ultra settings which is why the minimums are so low compared to the 290. A 4GB 380 that the op would purchase should show much better min, proportional to the difference in averages. Plus, according to the builds outlined by the op, going with the amd build wouldn't save near enough to upgrade to a 290/390 anyway.
http://www.hardwarepal.com/shadow-mordor-benchmark/




http://www.hardwarepal.com/shadow-mordor-benchmark/
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