I didnt leave mine on long enough by themselves to notice any drop.... (as I put then on- used them- and then removed them- its so easy to put in and off-- that I took them off).
BUT--- there was no increase in sound from the cross bars being up there themselves.
No change in mileage with the crossbars installed. The mileage does change a bit with the cargo box on though. The noise is a lot less with the cross bars removed. I only install them when I need to carry the cargo box.