There have been reports that a lot of developers have been working closely with AMD on upcoming titles because of DX12 / Vulkan's similarities with AMD's Mantle API, but a lot does not necessarily mean most of them and working closely does not necessarily mean that they will be optimising the game for Radeon.
"A lot" of developers will probably also be optimising their code around NVIDIA GPUs since they know that most of the people playing their game will be using NVIDIA GPUs and hence optimising their game for NVIDIA will likely have a more positive impact on their reviews i.e. there is more incentive to optimise for NVIDIA.
The likelihood is that after DX12 / Vulkan becomes commonplace in modern games, it will be the same situation as any other generation of games, that is that some games will perform better on NVIDIA whilst others will perform better on Radeon. However, I do think that DX12 / Vulkan's similarities with Mantle will perhaps result in a greater proportion of DX12 / Vulkan games performing better on Radeon than we are seeing in DX11 but since NVIDIA is more dominant in DX11 anyway, we might just see that Radeon balances things out a little more or pulls ahead a little in various benchmarks with the new API rather than jumping miles ahead of NVIDIA. We won't know for sure until games start coming out.
With that speculation in mind, I do not think that replacing your NVIDIA card with an equally performing Radeon card is justifiable. Both the GTX 970 and the R9 390 will be sufficient until the next line of GPUs come out, so you may as well just wait until then. Getting a 390X may be justifiable though, because even if Radeon doesn't see the expected performance boost with future games, you will have upgraded your GPU anyway so nothing is lost. However, I still recommend just waiting until new GPUs come out, unless you have the money to spare of course.
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