Well this is really embarrassing, but I made a huge mistake installing the Witcher 3 on Windows 10 preview..
Since the game has been released, I've been playing on Windows 10, and while it's performed decently, it has been suffering from some minor hiccuping or blips; especially in shadow dense areas.. I troubleshooted the hell out of this, and came away with the impression that it was likely a driver issue since I am using SLI..
But last night I thought I'd install Windows 8.1 just to satisfy my curiosity if there was any difference in performance. So I did, and my God, the difference is practically night and day! All of the strange performance problems I was experiencing with Windows 10 have vanished, and the game now runs perfectly smooth on full ultra settings, plus all Gameworks effects set to the max!
I was wrong to think that Windows 10 was ready for prime time use. It's not, especially if you want to play the Witcher 3. So I'm posting this to alert anyone thinking of installing Windows 10 on their primary PC and playing the Witcher 3. DON'T DO IT! Use Windows 8.1 for the best experience!