Intel says it tested 100 pieces of the i7-7740X and found that the 23 percent best chips ran Cinebench R15 at 5GHz with 1.205V. Another 53 percent achieved it with 1.215V and the 24 percent worst chips required 1.341V.
VideoCardz notes Intel recommends liquid cooling for Kaby Lake-X and Skylake-X processors, no matter if they're overclocked or not. Head over to the site to see some performance charts from Intel.