On a typical non-VSX bare-metal firewall given the limited blades you have enabled, leaving SMT/Hyperthreading disabled would probably be the right call; mainly because there would be a large percentage of traffic being fully accelerated by SecureXL. Most of the load would be on the SND/IRQ cores and SMT/Hyperthreading doesn't really benefit those cores much on a regular firewall, and can actually hurt performance in some situations like this: Firewall priority queues setting
But since VSX is more or less a process-based implementation of the inspection that is typically handled in the kernel on a regular firewall, I'd think that enabling SMT/Hyperthreading for VSX would actually be helpful assuming you have plenty of RAM. Whenever there are numerous process threads all competing for limited cores, SMT/Hyperthreading can permit threads to jump on a core while other threads block waiting for some kind of event. Since it is a gateway and not an SMS, there is little disk I/O interaction and the usual recommendation to leave SMT/Hyperthreading off on a R80.10 SMS is not relevant here.
This is a bit of an educated guess and I'm definitely not a VSX expert, so I would welcome input from someone at Check Point on this topic with more knowledge of VSX internals.
Second Edition of my "Max Power" Firewall Book
Now Available at http://www.maxpowerfirewalls.com
Updated 2023 IPS/AV/ABOT R81.20 Course now
available at maxpowerfirewalls.com