FWIW, humidity does play a part in airplane performance (both FS and model scale), but its effect is marginal compared to the effect of pressure altitude (elevation) and temperature....

In FS airplanes, pressure altitude and temperature are factored into performance calculations, but humidity is ignored, other than on a theoretical level... If it had a significant contribution, I am sure some bureaucrat in the FAA would require its inclusion in performance data.... I never met a bureaucrat that could resist expanding his empire...

Cheers!

Jim