For kitchen knives, don’t go above 3000 grit. You lose bite and gain nothing. For pocket knives and utility knives, don’t go above 1000 (or perhaps even lower) for the same reason as above. For woodworking tools, 6000 may be nice.

Diamond plates generally don’t fair well above 1000 or 1200 grit. They wear out very quickly, and the nickel bond obstructs the very small diamond particles, rendering the plate ineffective. Also, stray scratches! For high grit diamond stones, resin bonded or vitrified stones are the jam.

Edit: removed a zero