got the same issue with each 5800, either single, standalone or ClusterXL (especially with LSM mode) as well as with 5800 running on HA VSX with either R80.x or R81 (now R81.10).
8GB is a joke on that appliance, at least 80% of util. each minute each day all day round hence the 16GB is a bare min.
I did multiply RAM upgrades to that particular chassist and tell you something, 32 GB of RAM to this appliance cost you 33 GBP and modules are: Check Point CPAC-RAM16GB-5000 16GB 2Rx4 PC3-12800R DDR3-1600 Memory DIMM
I've purchased those RAM in UK and on eBay and seriously - HYNIX RAM works like a charm giving you 10-15% of RAM usage when either appliance setup works on 32GB RAM.
good luck with HW upgrade which for those who "CAN" open the chassis takes 5 minues 🙂
Jerry
Jerry