Thanks for the advice, and well taken.
My FreeNAS box has ran happily for 6 years on its' present 16Gb of ram, and recently I've noticed that it has been near enough maxing out. But that's only because I have looked at that time. I just thought it might be a bit better with another 8gb, but again maybe not required
First of all, reading RAM usage is tricky when it comes to FreeNAS and Linux in general, mainly because RAM does not look like it gets freed up when not in use. For your concern I point you to SWAP Space.
So what you can do to figure out if you need more physical RAM, check the SWAP file size. If you are using the SWAP file then you have run out of RAM. But let's temper that statement a little bit with it does depend on how much SWAP space you are using. If your SWAP usage is less than 500KB (I pulled that number out of my butt and it's just a value where I would personally be concerned), I wouldn't worry about it much, but if your SWAP usage in MB range and often, then you have a hardware limitation problem and should either add more RAM or possibly look into any jails/plugins you have to remove/disable what you can.
If you have 500KB or less of SWAP usage then adding RAM would basically make cached files (files accessed frequently) available quicker. There is more to it than that to be honest with you but making your system overall faster, generally it will not but it does depend on your use case. I'm sure there is a lot you could read about RAM shortages with FreeNAS, look up "SWAP space RAM FreeNAS" on the internet or something like that and I'm sure you will find something, it could take a while to filter through some garbage.
I will always promote adding more RAM if it's available but in your case since your system has been working for such a long time without complaint, I'm not sure I'd add any more without good reason. I'd save the money for next set of hard drives you will need to buy.
Also, I run my FreeNAS on ESXi 7.0 with 16GB RAM and it runs great so you are not the only person running 16GB RAM. I'm only running Plex in a jail and I do not use any SWAP space so maybe you really don't need the extra RAM.
Good luck.