With regard to the original question: I wouldn't be too concerned about Nvidia's DX12 performance unless they start showing poor results in games made with the major engines like Crytek, Unreal and Unity.
Many of the DX12 games so far have been very poor and although I gained a few FPS in DX12 Deus EX Mankind Divided on my 390X, it was an unplayable stuttery mess. Whereas Nvidia lost performance but it was smooth, so a better experience.
The developers who don't have the time, experience and money to implement these lower-level APIs were supposed to stick with DX 11.3 not shoehorn the new APIs into their own game engines for marketing reasons. Alternatively, they could use one of the big engines once they are ready, that was the whole idea after all. The only good implementations so far are Doom and Ashes of the Singularity and maybe Gears of War 4, at least for Nvidia, going by this.
I recently bought a GTX 1080 myself without any worries about this as I couldn't wait for Vega. Sorry AMD, you took too long; things have been very smooth indeed with my new Dell S2716DG except for some slight stutter in the Witcher 3.