I had the GTX 1080ti paired with a Core i7-4930k and there were no problems to speak of. The 4930k may have 6 cores/ 12 threads compared to 4 cores/8 threads with the 4790k, but the single core performance of the 4790k is a cut above the 4930k. 10%-15%, clock for clock, to be somewhat exact.
The only games where you'd pick up more FPS with a better CPU are CS:GO, GTA 5, or Black Ops 4, but you'd already get well above 120fps with the 4790k and a GTX 1080ti at 1080p. The 4790k with the GTX 1080ti would do well at 4k, and you would see NO improvement at all at 4k if you upgraded the CPU.
The real bottleneck you face is with your monitor. If you're gaming on a standard 60hz, 1080p display then the bottleneck is the monitor inability to display more than 60fps.
In gaming benchmarks, The 4790k keeps up with the Core i7-6700k, and it's just shy of the Core i7-7700k. The 4790k slightly edges out the Ryzen 7 2700x for the most part. You can pick up a few more FPS with the 8700k or 9700k but that only happens when the framerates are already high. The 8th and 9th Generation processors are still using the Skylake architecture, but the refinements of Intel's 14nm process has allowed Intel to pack more Skylake cores into a die while making clock speed gains. The IPC difference between Haswell and Skylake is 5% to 6% which is rather small. The only thing that can be said about Coffee Lake is the Memory controller is strong so you should have no trouble running the XMP profile with 3600mhz cl16 or cl15 RAM.
Personally I wouldn't upgrade until after Zen 2 or Intel finally moves past using 14nm++ Skylake architecture.