I've pretty much accepted that there is going to be the occasional artifact in Witcher 3, but I got it down good enough where I can go 30 minutes or an hour without seeing one, and when I do see one, I don't see another one for awhile. It could very well be a regular graphics glitch, because some of them I'm seeing at stock clocks as well. Every now and then then there is a large black rectangle that is about the size of Geralt that will appear to his left or right side. I'm convinced 100% its the game, or the drivers causing this. FYI Witcher 3 has never crashed on me once. Been playing the game from 1.01 up to 1.08. And I've played the game in 2 hour sittings many times. If better FPS means seeing an artifact once every 30 minutes for .1 secs, and the game doesn't crash on me then that's a good trade IMO. That's my definition of stable enough.
I played in 7 hr blocks with earlier patches and there was no instability.