It's nice to know I'm not the only one with these issues.
I have 2x Gigabyte G1 GTX970s in SLI with an Intel Core i7 4790K on three 10:16 monitors at 5040x1050. I can run GTA V almost maxed out at a stable 60FPS. The Witcher 3 I get between 30/40 fps at rather medium settings, taking the hair thing off, ofcourse
I also noticed that when I use the "Maximized" option I get about 50 to 60% GPU usage on both cards. When I set it to "Windowed without Borders" I get 99% GPU usage on both cards but absolutely no difference in performance and FPS. So I believe that both cards are still just under used.
I also noticed other weird stuff like when I stand on grass, if I move the camera arround, flowers and nicer grass just appears and disappears constantly right arround the character. Realy strange!
And the final thing which probably has nothing to do with graphics but I have to hit X (of my XBOX controler) like 50 times to skip one cutscene. It just scips a few seconds forward. Realy annoying.
Anyway, uninstalling the 352.86 drivers first thing tomorrow and getting back to the 350.12 drivers. If The WItcher 3 works better with those, great. If not, back to GTA V for me and no TW3. It was just a gift with my GPU's anyway. But it is a shame, the game looked promising.