Intel says that CPU Core i7-7740X will easily work on 5 GHz
Very soon you can buy Intel Kaby Lake-X and several models Skylake-X.
In the meantime, you can find overclocking results for some models. More specifically, with statistics, which allows us to understand approximately what frequency can be achieved with a particular CPU.
Thus, the model Core i7-7740X operating at frequencies of 4.3-4.5 GHz should easily overpower the bar at 5 GHz. However, it will need to raise the voltage to 1.2 to 1.34 V.
As for cooling, in this case, it is desirable to use, ISO. In the diagram below you can see the temperature of the CPU at the factory with different frequencies. Green represents data for the unknown air cooler, red for ISO Cooler Master Liquid 240, blue for GSO Corsair H110.
A similar diagram for the Core i9-7900X as follows.
In addition, the results of testing these CPUs.
Note that all these data are given in the guide for peer reviewers provided by the company Intel.