You can put a 7700k in there. The z170 chipset is compatible, but you may need a bios update. I’m running a very similar setup I have the GA-z170x-gaming 5 with a 6700k and have been through this several times in my mind. I ended up just overclocking the 6700k and deciding to wait for the 10th gen intels.
If you overclock the 6700k to 4.5ghz, you will likely see performance on par with a 7700k. 4.5ghz is a very safe and easy OC for the 6700. For reference I’m running a 2080 super on this rig and have had no problems ripping through all games, even in 4k.