Is it worth refilling these fire extinguishers or should I just throw out

I used to be a fire extinguisher repair tech. This unit should not be refilled even though it is possible with the right equipment. The plastic/nylon heads will develop cracks and leaks over time. Buy a new unit with a metal head. Those can be refilled and will last you a long time. Amerex, Badger, General, Ansul are all decent brands.

I might add that the primarily "fire rating" of wall and floor/ceiling assemblies is to allow occupants to safely exit the building, not to keep the building from burning down. The Type X gyp board affords some protection because the inherent moisture in the product can slow down the fire through your wall or ceiling. With these assemblies, you are basically looking at a way to escape the situation before your obvious portal to Hades has the structure tumbling upon you. Perhaps you should also look into a fire suppression system--not sure what the initial source of flame is (what might be burning other than building components), but sprinkler heads tied to a detector might be a good idea.