Overclocking the Core i9-9900KS is a balancing act between adding too much power - frequency and voltage - and keeping temperatures in check enough to inhibit throttling. Adding a second fan to the Noctua cooler and raising both frequency and voltage just enough to keep everything in check, we settled on a 5.25GHz all-core speed at 1.375V. Intel also lets you overclock automatically using the Performance Maximizer Tool, but we prefer the old-fashioned manual approach.
The chip actually boots at over 5.30GHz and runs the benchmarks supposedly fine, but a look at the logs shows throttling taking place, reducing performance compared to our 5.25GHz all-core overclock.
Not huge gains, because the chip is already running pretty damn high, and we'd probably advise against it unless you have some excellent liquid cooling in place - the twin Noctua fans have to run at relatively high speeds to keep the chip from throttling on long-term load.
It's worth knowing that Blender system-wide power consumption jumps from an already-high 236W to 271W.