To be fair, Doom on OpenGL was running horrible on AMD, some tests where showing a 970 around 390X/Nano levels, the 960 4GB was better than 380, and even matching or over 380X perf, that was no normal.
Now with Vulkan its more where it supposed to be, with maybe a small plus on AMD side, i think that was more horrible AMD OpenGL drivers than anything else, thought im sure shader intrisic (with GCN extension on Vulkan) and async are helping AMD as well.
And Kepler was performing badly on OpenGL already, i think its the worse game ever for Kepler, and this is no a gameworks title, people has been claiming gameworks was doing this in order to force people intro buying new cards.
And what happens with Kepler in Vulkan its on the devs hands, if they dont take the time and money to optimise for it, there is no much that can be done.