I don't have any experience with the new 5900 model yet but are you sure it has 8 physical cores hyperthreaded to 16? That seems like a lot for that appliance level and Oliver Fink has not yet updated the specs for the 5900 here:
https://lwf.fink.sh/tag/tobias-lachmann/
Providing the output of "cat /proc/cpuinfo" to the website above would help. What code version are you planning to use, R77.30 or R80.10? That will make a big difference.
Assuming it does have 8 physical cores it should have a default 2/6 split that extends to 4/12 with hyperthreading enabled. Systems that have a lot of PXL/F2F traffic are good candidates for Hyperthreading, depending on your IPS Profile (especially if using Default_Protection) you may have lots of traffic being accelerated and handled via the SNDs in which case enabling Hyperthreading can actually hurt performance. Tough to say what will happen until you put it into production and see how traffic is getting handled with "fwaccel stats -s". Enabling Hyperthreading is not necessarily a no-brainer.
Based on your post, I'd say try a 3/5 split w/ no hyperthreading initially and assess traffic acceleration levels; any time multiple 10gig interfaces are involved you may need to enable Multi-Queue and/or increase SND cores.
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