I think it's pretty hard to recommend a barrel length.
First a 28" barrel on one gun swings different than a 28" barrel on another gun. Part of this is total gun length and weight, if you pick up 10 different o/u's with 28" barrels they may vary by as much as 2-4" overall and probably 1-2 lbs in weight, maybe more.
Balance point, total weight, barrel length, and most importantly the shooter him/herself all effect how guns swing. If you naturally swing very fast you'd be amazed how long of a barrel you can get away with. On the other hand if you swing slow just the opposite. Balance point is huge here too. If the balance point is too far forward or back it effects swing speed as well.
Generally I consider a 30" to be a pretty good compromise for all sports and most guns. 28" can get pretty whippy on some of the new shorter/lighter over and unders. 32" on the other hand can get pretty heavy. The 2" itself isn't a big deal, it's that usually that 2" on the end of the barrel changes the balance point a lot which is what really slows down or speeds up the swing.
Sadly, that doesn't really help much, since the largest factor is the shooter him/herself, it's very hard for anyone but you to decide which length is right. That said it's easier to slow down a gun that moves too quick (add weight) than it is to speed up a slow gun.
As for the sight lenght.....I think it would be pretty hard for just about anyone given two exact guns altered in weight so they had the same balance point and total weight to tell you which one is a 30" and which is a 32" just by looking down the rib. As for aquiring the bird, I can't say, I shoot both eyes open and hold off the top of the house and look under the barrel for the bird as it comes out of the house.