High specific humidity does adversely affect engine thrust - due to the lower density of the air. This has a much larger effect at N1 than at EPR (obviously signficant only at higher temps, since you need high temps to have high specific humidity).
Ummm,, what?!?! You need High temps to have high specific humidity??
The standard is BASED on 80% to 34% rel humidity. Looking at the calculations, the variable in the calc for humidity is multiplied by .0621, and the humidity ranges between .0008 and .007. Therefore .0621 * .0008 to .007 is meaningless as an additive to the performance calculation. That is why is is negligible, and not added to any performance software, even at any altitude.
Temperature governs, plain and simple by a vast margin.