I hadn't seen the new Mac Pros before and they look awesome (if pricy!). The FirePros will be able to crunch GFN, GFN-WR & PPS-Sieve with the stock apps on PrimeGrid. CPU will be fine (obviously) for any LLR-based tasks, along with the rest.
I can't comment on the performance you should expect as I don't have experience of the AMD cards, but it *should* just work!
Proud member of team "Aggie The Pew". Go Aggie!
3073428256125*2^1290000-1 is Prime!