Overclocking the Core i7-8700K is relatively easy because of its unlocked multiplier. All you have to do is set the desired multiplier in the BIOS and you're good to go.
Our sample reached 5.0 GHz easily, with just a small +80 mV voltage increase, which also helped keep air-cooling temperatures low enough to ensure the CPU won't throttle. 5.1 GHz required a lot more voltage, in the 150+ mV range, which, due to increased power draw resulting in more heat output, overwhelmed our air-cooling solution. I quickly tested with a 240 mm AIO, and the CPU was indeed stable at 5.1 GHz in that scenario at decent temperatures. Achieving 5.2 GHz stable was not possible on our sample, not even when watercooled.