As to your question of "Is it critical to replace immediately? ", I don't have specific experience with this on the NSX, but I do with other cars. The knock sensor provides feedback for the ECU to set the timing advance. If the sensor is bad, it is possbile for the ECU to advance the timing too far, risking pre-detonation and possible piston damage. If however, when it throws the P0325 code the NSX ECU goes into a somewhat safe mode and retards timing , then there is no risk, just reduced power.
Does anyone know for sure how the NSX ECU handles this case?
Not knowing the specific answer, the "thoughtful driver safe mode" would be to avoid heavy acceleration until the problem is fixed.