There's a thread of high quality on the possible benefits of a DirectX 12 adaptation here:
http://forums.cdprojektred.com/thre...le-support-post-release?p=1557157#post1557157
This thread is about the system requirements of the game as it stands, which does not include DirectX 12 support.
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The stated minimum is a 7870, and the 7850 is not quite its equal. So at a little under the minimum, I think you'll succeed with some combination of low-medium settings and reduced resolution. Comparable to console performance, which as we have seen looks like it will be satisfying for most of us.