Dude, I have a 5820K @4.25GHz and one plain old Gigabyte GTX 1080 and I do all sorts of 4K streaming, 4K screen recording (Bandicam), 4K gaming max graphics, music editing and photoshop with ZERO issues and no lag. I use 16GB of Corsair Vengeance LPX DDR4 @3GHz at stock timings. I overclocked the RAM timings some to 13-15-15-33 (vs 15-17-17-35) but saw no difference no matter how hard I pushed the system to justify the increase in memory errors reported by memtest86+ (despite being within safe error margins). An SSD helps loading times and I turn off Windows Page File so all the RAM is fully utilized.
I dunno what you're thinking about such an expensive system, but believe me... Intel's latest offerings and Nvidia's 1080ti are full on Demolition Man murder-death-kill for anything.
So far, despite maxed graphics all latest games are very nice FPS, even horribly optimized and demanding games like GTA5 or Just Cause 3 run 50 FPS steady with my system. A few gig 1080p video takes seconds to convert and 4K video editing takes an average of 4x longer, still only averaging minutes to convert 50+++GB videos.
Just get a 5820K on sale with a single 1080 and save hundreds for great performance. Remember that 99% of the things you want in a computer simply decrease TIME and actual performance is just a measure of how long you wait for something to finish. It's only in live action things like gaming that performance drops are annoying, and believe me, a system like this will not lag on these cheesy half-a**ed games that come out these days.