POWER CPU cores typically have N-way hardware threads, like 8 or so usually. So you're probably looking at something closer to 32 hardware threads per socket. The original Talos offered 12 core variants (96 per socket!) but had no dual socket config. Also, the memory bandwidth and cache on these things is insane even compared to top of the line, absolute highest-end Xeons, so there are many factors besides raw CPU count.

I don't know if those numbers are upped with a POWER9 CPUs (I wasn't even sure if POWER9 was really shipping yet?)

It is; the question is, can you put the 24-core chips from IBM in the same socket? For that matter, what on earth is the socket name for POWER9 (or POWER8)?

After some more looking around, talking to someone on reddit.com/r/openpower, and looking at the new site information; the thought is that these are 50 mm × 50 mm Sforza sockets and the 4-core CPUs are 12-cores with 8 cores disabled.

What is their performance?

IIRC they're a bit better than an Intel core on a single threaded load with some variance depending on the workload. But going from 1 to 2 threads per core increases the Power 9's throughput by 80% instead of the 20% you expect with an Intel core. And you can go up to 8 threads per core for something like 120% the total throughput of 1 thread. All those numbers for threads per core are also highly workload dependent. The more cache misses the more SMT helps.

Can you give a source, or was this personal testing?