From all that I've read, the biggest winner in the DX12 sweepstakes might be AMD with the hardware we have available now. If nothing else, the increased load spreading for draw calls should help AMD's drivers which under previous DX versions have always struggled versus Nvidia's very optimized drivers. Nvidia has apparently gotten very good at distributing certain DX11 and prior rendering workloads across multiple cores, whereas AMD tends to weigh more heavily on single core performance.
Especially if you have an AMD/AMD box---you have the single core deficient (vs Intel) performance of an AMD cpu and drivers from AMD that are less multithread optimized than Nvidia. DX12 would seemingly give the most measurable advantages to those setups---theoretically decreasing the emphasis on single core performance for both videocard driver and DirectX.