Vulkan does hardly anything for a GTX970 in Doom, though.Doom Vulkan on a 4690K and GTX 970 is really rather nice, i thought, DX12 is a bit 'meh' Vulkan is what DX12 promised to be.
Either way, this is not how it works. AMD cards see big gains with Vulkan in Doom because the original path is OpenGL, something AMD notoriously has lackluster driver support for. Vulkan over DX11 is not going to see anything like the same thing and will be generally identical to DX12 in terms of improvements. Vulkan and DX12 do not really differ much at all in terms of advantages. They are two sides of the same coin. As always, a low level API implementation and its results are going to depend a lot on effort, engine and specific bottlenecks for a given application. If you think Vulkan in Doom is somehow representative of how Vulkan will work everywhere, you obviously dont at all understand what DX12 and Vulkan actually are.