Well, on an arrow with decent mass, like the 2315, you won't see a huge difference between 4" vanes and 5" feathers. The arrow's weight will have enough momentum to maintain downrange speed in spite of the extra drag.

Put those 5" feathers on a light carbon arrow though, and those big feathers will have a lot more effect. Less mass, less retained speed and energy downrange, so it's more susceptable to drag. Another way of saying light carbon arrows can't even penetrate AIR as well as a heavier arrow.