I shoot 5/16 shafts at mostly 26" long, sometimes shorter shafts for my plains style bows. I fletch them with 4" feathers. If I were shooting 28" 11/32 shafts, I would fletch them with 5-inch. The technical reason: it just looks better (IMO).
Maybe for me its similar to the old adage that says, shooting a 23/64 shaft through a kids bow is like shooting a broom handle through our adult bows.
My arrows seem to correct themselves fine past 10 yards and I think that there is less air drag so, maybe they're faster. They definitely are quieter. Again, no science here. Just feels right. I don't chrono. I'm not much of a technical nut when it comes to our primitive bows.