Overclocking a processor takes a lot of guesswork–especially when it's a freshly-released CPU like Intel's Core i7-7700K. From figuring out the target voltage you need to picking out an adequate cooling solution, there are tons of variables to account for. The minds over at Asus's EdgeUp blog have done a bunch of research on overclocking Kaby Lake CPUs, and have come away with some interesting results. Cranking your Kaby Lake CPU to 5GHz with just air cooling is within reach—if you're lucky.
The people at Asus found that about 20% of Core i7-7700K CPUs were stable at the 5GHz mark when put under AVX workloads. Even then, the chips begin to run out of thermal headroom and throttle around 1.35V—something that's reasonably consistent with our own experiences here. Asus did have some luck with improving cooling by de-lidding the CPU and swapping in a more conductive thermal compound. This operation allowed Asus' testers to reduce core voltages at a given overclocked frequency.
Asus noted in its breakdown that DDR4-3600 RAM appears to be the upper limit for plug-and-play overclocking. In our review, we were able to use DDR4-3866 RAM without major issues. Maybe we got lucky, though.
It's also worth mentioning that Asus is porting its Windows-based Thermal Control Tool to its Z270 boards' firmware. The Thermal Control Tool can help chips plugged into those boards behave better at the upper limits of voltage and frequency by clocking the chip down when it hits a certain preset temperature limit. Once it drops back down far enough, the CPU will clock back up. That should help keep the chip running within safe ranges and potentially extend its lifespan.
Asus's guide is extensive to say the least, and worth taking a look at if you're hoping to squeeze some extra juice out of your CPU.