You guys expecting Coffee Lake (much less, Skylake-X) to be 5GHz are going to be disappointed. The 7700K was already outside of its ideal voltage to attain its stock clocks. Reaching 5 GHz on even 6 cores would be looking at 200W+ TDP...
It's possible...
I mean, it would be Intel's third attempt at client Skylake, plus a better process and physical implementation, plus the aid of delidding + CLU and a properly overengineered motherboard with a beefy VRM... Coffeelake's die size is going to be obviously bigger than Skylake/Kabylake, so that will help with heat transfer. Voltage required to get there will probably be similar to Kabylake, or a little better considering 14nm++ improvements.
If it doesn't hit 5GHz under these ideal (if not punishing considering the long term) conditions it'll be very close, or it'll be just like the 6700k with a little % actually getting there and not hitting a voltage/thermals/frequency wall based on die quality. We've had >200w CPUs before when overclocked, >4GHz Bloomfield (i7 9xx) was probably in that ballpark and it wasn't a problem getting that cool enough. You just had to deal with a PC doubling as a space heater
Skylake-X on the other hand... nah. No way it's going to overclock like Kabylake. Intel's server dies have historically overclocked worse than their mainstream dies, and this time you not only have a bigger L2 that could be a problem, but also the AVX-512 hardware that's even bigger and more power hungry than what's on client Skylake cores. Sure, you have 14nm+ as the base here helping... but stiIl, I'd be surprised if it overclocks worse than Broadwell-E. Probably similar top best case clocks (4.2-4.3GHz)
Having said that, it's going to be more of the same, and it's just Intel milking Skylake as much as they can until they get their next gen core out of the door in a few years or whatever. That 6 core Coffeelake is LONG overdue for the mainstream socket.
When was the last time Intel operated like this, staying so long on the same base architecture? The Netburst days with 4-6 years of polishing that turd through node shrinks/improvements and the natural single core -> dual core transition?