It's a win for gamers.
DX12 isn't just a win for AMD, sure, it helps them due to native multi-thread rendering and lower API overhead (which NV already has to some extent on DX11). But the features in DX12 help all hardware.
Overall, DX12 levels the playing field because it removes the major weakness of AMD in DX11, single-thread rendering and lack of direct hardware access & scheduler which GCN is designed around (for consoles & console-like API).
When you talk about winning in DX12, you have to be specific and talk about the features.
The optional features like Async Compute could help AMD if it were used extensively. Likewise, FL12_1 could help NV if they were used extensively.
So its game-dependent. I haven't heard anything about SC using Async Compute heavily for it to be a win for AMD. If anything, the game will run great on NV as well due to working with NV.