If we are talking about GTX9xx and R9 3xx, by the time DX12 became mainstream, you are already on your way looking for newer and better GPUs.
It does not matter if it is nVidia or AMD.
If the rumors are true about having a huge performance jump, even Pascal and Polaris will make all current GPU near obsolete already.
No need to make a fuss on DX12 topic for now.