I've no idea how much the 390(not X) 4GB will cost, but unless it has a very large number or shaders shut down, it has a good chance of slaughtering the TX in Vulkan / DX12 in games not demanding huge amounts of VRAM.
When Mantle was coded exclusively for GCN it couldn't even slaughter Nvidia with DX11 and all of its overheads so how do you come to that conclusion?