Obviously temperature and humidity have an effect on engine performance. Is there a rule of thumb or chart that can give some insight on to what is being lost RPM wise as these two variables rise for a given EFI motor? Carb motor? Any fast and easy reference? Is dropping a pitch in prop size recommended to get back some RPM?I have noticed only about a 100 RPM difference from the hottest days to the coldest (40 degree F temperature delta) with a 575 EFI.

i dont know of any chart,but i know efi compensates for heat&humidity,but a carb can not.i would say a max of 100 rpm difference is not bad.it would be worse on a carb engine.

humidity doesn't have a huge effect on power until it gets to the extremes but air inlet or charge temp is a big deal. generally speaking, a 10 degree increase in charge temp is equal to roughly a 2 % decrease in power . dyno correction factors presume this is linear but in the real world the temp tends to effect the way the motor runs at the top much worse than it does at the bottom which is a charge density issue. injection deals with this much much better than a carb does. the single best thing you can do for your motor is to keep the airflow thru the engine compartment high and as close to ambient as possible.

its also the reason why when people want to publish big dyno sheets they often do nothing more than move the air temp sensor someplace hot so the dyno thinks the air inlet temp is 95 deg or something and automatically generates a giant correction factor and a hence a giant corrected HP.