The Talos computers are only compatible with SMT4 chips (aka: Sforza chips). You can't buy an SMT8 chip for Talos.

SMT8 chips are also known as "Scale up" chips. The Summit supercomputer was made with SMT4 chips by the way.

I never used an SMT8, but from the documents... its really similar to two SMT4 cores working together (at least on Power9). SMT8 chips are a completely different core than SMT4 chips, with double the execution resources, double the decoder width, double everything. SMT8 is a really, really fat core.

The scale-up chips (used in the E950 and E980 only) aren't the only SMT8 chips. The scale-out chips (Sforza, Monza, LaGrange) can be fused as SMT4 or SMT8, but IBM doesn't appear to sell SMT8-fused chips for other parties.

If a given IBM server runs PowerVM, it's SMT8. You may find this table of mine helpful (assembled from various sources and partially inferred, so accuracy not guaranteed, but it represents my understanding): https://www.devever.net/~hl/f/SERVERS