I haven't had the opportunity yet to hunt flushed birds. Walked up a few quail as a kid is all. I currently am limited to shooting Dove out in the open. So far, I've been happy with a 27" O/U for this. I also use it for shooting clays as a novice. I have scared up a few birds like woodcock and quail in the woods walking through. They always seem to quarter away for the most part. Not much to swing at. Have never seen grouse in the wild. I understand they are fast at the flush. Do they generally take hard angles requiring a great deal of swing? I would imagine if you are hunting a bird that does not require much lead, the longer barrels would loose some of their advantage for those who rely on the momentum of longer bbls to follow through on the shot.
