Do you not consider the freedom aspects (e.g. sources to the secure boot implementation, no x86, etc) as advantages in favour of Talos?

That's definitely nice! Any other advantages?

The Power9 should be more performant than EPYC/ThreadRipper and CAPI gives you higher memory bandwidth to a GPU.

Are there any benchmarks that I can look at for this?

Here is a comparison of POWER8 vs Intel Xeon http://www.anandtech.com/show/10435/assessing-ibms-power8-pa...

Huge memory BW, CAPI2, 8-way SMT, 120MB L3 cache, L4 cache, compression and crypto coprocessor, etc...

They are selling Scale-Out processors here, which IIRC “only” have 4-way SMT (which will, given IBM's track record, still actually give a ~2.5x improvement over no SMT, unlike <1.5x for SMT2 in Intel/AMD)