If anything should convince you DX12 is going to favor GCN, it should be this:
Devs have praised GCN for DX12 publicly on tech forums, they even go so far as to highlight its ability for fine-grained execution of async compute beyond 1 thread/queue, to saturate the shaders.
But whether it has an impact in games, will depend on the game. Just because its a DX12 game does not equate to it using a lot of async compute features. It could simply be the low lvl CPU overhead reduction being employed.
The Starswarm demo favors NV, while the draw call 3dMark bench favors AMD. Basically synthetics tell us very little, we can only judge once a bunch of DX12 games hit.
Interesting times ahead, but not least because I am looking forward to Battlefront & Deus Ex!