I know nothing about 4K gaming except what I read here and see on YouTube. I've heard Witcher 3 at 4K is tough. Only thing I could suggest are obvious things like turn off hairworks and turn down AA. You think it's tough now, imagine if you upgraded from ultra to uber settings. I game at 1080p, but will upgrade in the future to 1440p.
Corsair Ax1200i - You own this? (http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=300). Wow. You appear to like Corsair, how I like EVGA. Two Samsung 850's? Are they in RAID 0 or just independent drives? 32 GB of RAM, nice. I'm building a new machine now and I honestly would have been more than happy with the 4790K, but I want to add more than 32 GB of memory; hopefully 64 GB. Maybe I could pick your brain.
I started out seeking the 5820K but I'd like to eventually SLI 980 Ti's and the thought of combining that with two PCIe drives... Well I was worried that the 28 PCIe lanes to the CPU might be limiting. Then I was thinking about getting the 5940K for 6 core 40 lanes, but I'm also considering purchasing the 5960X because microcenter sells the extreme edition processor under cost for walk-ins. I also need to select a motherboard, if you have any suggestions.