As an afterthought, I believe the question arose recently as to whether single engine power assurance checks on twins were adversely affected if conducted in high humidity conditions.
Logically the torque output of a free power turbine engine will be affected whenever the gas generator section is N1 limited, since high humidity will reduce density and hence the mass flow. This is in addition to any reduction in rotor performance. As there was no easy method of establishing absolute humidity in flight, there was no answer to the question.