14nm 6700k overclocks to about 4.5-4.6GHz and then it becomes problematic to scale higher, either because of a hard wall on your particular sample, or voltage, or heat generation becoming a problem. Delidding+CLU helped. At least it overclocked better than 14nm Broadwell... what a step backwards on that regard vs Haswell.How do you justify claiming it can do 5GHz? So far the HEDT 6c and 8c have had a distinctly lower OC potential than mainstream 4c, even on the same generation (mainstream on average being at least one generation ahead).
14nm+ 7700k can do 5GHz prime stable when delidded and using CLU to provide decent heat transfer to a decent heatsink. It's your very same 6700k implemented on a better process with some improvements on the iGPU side. See how process improvements and a better physical implementation help with clockspeeds and heat generation?
Coffeelake will be built on 14nm++ which features higher transistor performance at similar voltages than 14nm+. It's the same Skylake cores implemented on an even better process than the 7700k did, with the difference being that you now have two extra cores and some more L3 cache to feed the whole thing.
Seeing the evidence so far and Intel's by now quite extensive understanding of its Skylake architecture, it's not far fetched to expect 5GHz (or near 5GHz) on a decent heatsink when delidded (Intel won't start soldering its mainstream processors again now...) and using CLU just like on the 7700k. Overengineered motherboards won't have a problem supplying the power needed for those extra two cores. One can always get better cooling if needed.
HEDT has historically overclocked worse than mainstream implementations of the same architecture. HEDT dies are server/Xeon dies, those are much more complex and include lots of extra features... If you increase complexity, clock speed usually goes down. Coffeelake is a mainstream part, therefore this is a lean die without all the extra stuff that would go in a server part.
Yeah, it's a safe guess that 5GHz is a possibility with this.