I got my hands on a reviewers’ guide for the X299 platform. One of the chapters is dedicated to Core i7 7740X overclocking, probably the most interesting part of this noninteresting processor.

Core i7 7740X should easily hit 5.0 GHz

Based on 100 samples, the Core i7 7740X should hit 5.0 GHz at 1.205 V. In the worst case scenario it requires 1.341V to achieve this frequency. Some samples will hit higher frequencies (5.1-5.2 GHz), but those require higher voltage.

Overclocking on X299 platform comes at the price of high temperatures. The guide recommends liquid cooling for overclocked KBLX CPUs and SKLX CPUs no matter if they overclocked or not.

Green: air-cooler, Red: Cooler Master Liquid 240, Blue: Corsair H110

Some other charts from the guide you may find interesting: