well they're more effective at cooling than the AIOs used on these engineering samples, so capable of 4.6/4.7ghz.
Hexus were using a D15. They were hitting 100C.
http://hexus.net/tech/reviews/cpu/107017-intel-core-i9-7900x-14nm-skylake-x/?page=8
We'll start with the usual proviso: your overclocking mileage may vary and discussions with various partners lead us to believe that frequency headroom fluctuates significantly from one sample to the next. Our chip seems to be a good one and had no qualms about running at 4.7GHz across all 10 cores. Heck, it needed only 1.25V to make it happen.
Could frequencies go higher with more voltage? Probably, but putting 1.3V through the Core i9-7900X veins resulted in temperature soaring beyond 100ºC and automatic throttling. We swapped out our favoured Noctua NH-D15S in favour of an EVGA CLC 280 liquid cooler but even that couldn't cope with the increase in voltage.
Also doesn't look like they ran Prime95 or anything to really work the cores.
I will make a prediction. Running CPUs at 90C+ will suddenly become acceptable.