AMD gets a large improvement going from DX11 to DX12 because their DX11 driver is pretty crap. AMD gets a large improvement going from OpenGL to Vulkan because their OpenGL driver is pretty crap
While it's true that AMDs OpenGL drivers are performing rather subpar (just look at Doom benchmarks), one has to admit that the performance in modern APIs is really awesome.
I realized that when I swapped in my old R9 280: It outperforms my GTX 780 (which is usually MUCH faster) by at least 20% in Doom (Vulkan)! Quite impressive to see >60FPS@1440p in a modern game from such a old card. A GK104 card (direct competition back in the days) is getting nowhere near that.