Humidity affects engine performance in a similar way (i.e. reduced air density and thus mass flow) but there are additional complex effects on each component; specific heat (which changes the internal pressure-to-temperature relationships), and inlet condensation (which heats the inlet air as static pressure drops).
IIRC the published data are based on a fairly high standard humidity level (85%??) so if humidity is low, your performance will be better than book.
OTOH, at very high relative humidity, it's prudent to give yourself a little extra margin.