The i7-7740X has top of the line single core performance, but lags behind six and eight core processors in multi-threaded workloads. Anandtech have managed to push their CPU to 5.0 GHz stable on all cores. This could indicate better binning than the i7-7700K, but a quad core is still on odd choice on the enthusiast X-series platform.
https://www.notebookcheck.net/Kabylake-X-i7-7740X-overclockable-to-5-Ghz-on-air.237002.0.html
Even with the i7-7740X running at 200-400 GHz higher than the i7-7700K
WOW. 7740X@402-404Ghz!
Now that would be an impressive CPU, wouldn't it!
Thanks for spotting the error, the article has been amended to show "200 - 400 MHz."