I don't think 7920X will be a good choice at all for OC:I think the best chip for people who like to overclock from this year´s "generation" will be the 7920x, under condition it will come soldered. I would hazard a guess it could be OCable almost as high as 7900x (4,6+), cause only 2 more active cores, thanks to the possible solder not as difficult to cool, and overclocked on par with 1950x performance-wise (more or less, if 7900x can get ~2500 CB points at 4,5, then 7920x will no doubt attack 3000).
Granted, it will be slightly more expensive and less efficient/more power-hungry (but you never know, perhaps HCC dies will be for whatever reason superior in this regard to the LCCs, although i doubt it), but in regard to overclocking and top attainable clocks, probably way more fun than Threadripper.
- 7800X doesn't seem to consume that much less power than 7900X, as if those disabled cores couldn't be completely disabled. This would mean power consumption of 7920X won't be very good either.
- 7900X has quite high L3 latency, almost as high as cross CCX of Ryzen. 7920X is expected to have the same latency like 7980X, which will be more than 7900X. You are buying just 2 extra cores with worse latency.
- with practical upper limit of 7900X about 4.5Ghz (due to thermal issues), 7920X will be maybe 4.2Ghz for all core clock without delid
- its questionable whether 12-18C will be delidable, they will have bigger die
- at high clocks Skylake-X is inefficient, I'm not sure you want to have a case next to you with fans at maximum (even water cooling)
- I think you will get better performance increase with OC, due to this CPU having lower frequency where it is more efficient. Increase of 100Mhz brings less heat increase (per core) than with 7920X. Percentage wise you will get more performance for each 100Mhz, due to base being lower.
- use DDR4 4000 memory (instead of low latency 3600) to get good bandwidth as this CPU might be bandwidth starved (unlike its 6 channel Xeon brother)
- you may even get better performance than Xeon 28C / Epyc 32C with extreme OC (as they both have low clocks and are locked)
- you tune turbo 1-2 core frequency to be about 4.3Ghz (and downclock the rest, even below stock) and still may get ok performance for few thread applications
In my opinion the 2 most interesting X299 - CPUs are 7900X (high clocks, decent for gaming and work, can get performance like 12C) and 7980X (potentially crazy performance like Xeon 28C when OCed), but the price of 7980X playing against it.