On a 6000# vehicle, not sure I’d be able to notice a difference in performance or handling if I removed 100#. I certainly don’t notice a difference when my wife buys 200# of groceries for my family of 6.
As for the spare feeling like it only has 18psi, that’s because people tend to forget to check the pressure on their spare tires. I was taught at an early age that you need to check the pressure in your spare regularly otherwise it’s useless if you get a flat and it’s flat as well.
I find it interesting that some folks want to take as much weight out of their vehicle as possible including the spare, jack and lug wrench while at the same time, there is a thread on this forum where folks talk about carrying 200# or more of tools, spare parts and emergency blankets.
Just goes to show the wide range of folks that drive these types of vehicles.