it really depends on what you are doing with your computer.
The 1080Ti suggests that it's a gaming rig.
for gaming, 1800x would not be an upgrade. you will have lower FPS in quite a few games compared to 4790K.
there are very few games that will have perceivable improvement if you upgrade to 8700K. If you do things like streaming, yes it will be an upgrade.
The monitor is also plays a role. unless it's 144Hz or higher, the CPU is not that important.
If you do decide to go with upgrade:
1. make sure you get a decent Z370 board to have overclocking option.
2. i'd be happy to buy your 4790K for ~170$ + shipping