I don't know about the rest of the Cascade Lake-X family, but the 10980XE should top out around 4.7GHz. I got it to 4.7GHz (all core) at 1.165v and that's pretty much the ceiling. It will POST and even get into Windows at 5.1GHz, but anytime you do more than the most basic tasks thermal protection kicks in. Some extremely lucky few might see a bit more, but based on what little information I've seen from other reviewers on the topic, 4.7GHz seems to be about the maximum you'll get from these. From what I can tell, it should compete well if not edge out the 3950X, but obviously there are some major caveats to going with an Intel 10980XE over a 3950X.at this point im mostly interested in clocks and OCing but i think im going to have to wait for a few real overclockers to get their hands on them. Regardless i think im going to bump up from a 10 to 18 core. Just need to find out which of the 3 chips i want.
I won't say that the platform cost is necessarily higher. X570's do start out at a lower price but your scraping the bottom of the barrel. On the upper end of the spectrum X570 and X299 top out at similar prices. The 10980XE will cost you more for three reasons. You will need four sticks of RAM, not two. You will also need a much beefier power supply. The power draw on the 10980XE is far greater than that of the 3950X.