I've used 4, 5" parabolics for ages. Just in the last couple years I've cut back to a 4 3/4" feather. I can almost always get a second cut that works just fine.
My arrow/bow combo is always well tuned, but I prefer plenty of feather in addition to a shaft that's finely tuned.
Good tuning has a big impact on how the arrow reacts while it's going down range of course, but it applies mainly to the arrow as it interacts with the bow and it's reaction to that.
Once the arrow is clear of the bow the fletch must stabilize the arrow against outside interferences like wind or unseen obstacles.
I feel that it's always smart to err on the side of stability when assembling a hunting arrow. I gladly sacrifice the couple of feet per second it costs me (if that!) for an arrow I KNOW is going to impact straight, true and on the mark.