Does higher humidity result in greater performance? Better MPG?
You’d see no noticeable difference, not unless the greater humidity encouraged more use of your A/C in warm months and window defogger on cool months. Then humidity works against mpg.
This is the human factor, which is not what you are really asking. But you cannot separate the human factor from actual driving conditions.
I doubt that the effect would be noticeable, but humidity would reduce the maximum power available from the engine. The humidity (water vapor) mixed with the air reduces tha amount of oxygen that the engine can take in to burn with fuel.
I agree that you should pay more attention to accesories like A/C useage. In modern engines the air that is taken in is highly filtered, so I would say that the filters would filter out moisture. I would pay more attention to the sodium content of the air (If you live next to an ocean)because that can affect the paint on the car.
Car air filters do not remove moisture!
Some people will notice, by seat of the pants feel, that their car seems to have more pep on cold, damp mornings. I know I do. Suspended water molecules affect combustion in a few ways; it cools the intake charge making it more dense, it expands during combustion when it changes from a droplet to a gas thereby increasing the combustion chamber pressure and it slows the flame front during combustion which can improve combustion efficiency. All of those effects can contribute positively to the engine performance. Water injection was used on aircraft engines during WWII as a way to improve performance. Where already in a vapor state, it is not likely to be as beneficial.
Yes, higher humidity has a definite affect on performance especially of it goes along with lower temperatures. Both affect the air density. Drag race crews measure it in “particle count” and adjust their air/fuel mix (along with other variables such as temperature) for the best performance, adjustng it as the day wears on. If you listen to the races on TV you’ll hear the announcers state the particle count at the track.
On a stock family transport it won’t be as noticable, but on some you feel it.