OK, that clarifies your point.
For my thin & dusty dime's worth of opinion, I don't think a 28" barrel on an O/U makes that big of difference in follow through as opposed to a 26", but for me it makes a difference in handiness, storage, transport, and field carry. Besides that, if one is using extended choke tubes as many if not most guys do, they add 3/4"-1" to the barrel length.
I think the shotgun's overall weight is a much more significant factor in in follow through. My first string pump gun is a 26" Browning BPS, which because it's almost all steel, comes in at considerably over 8 lbs before I load it and put 5 goose shells in the butt stock carrier. I don't use it much for upland because it's so bloody heavy to cart around, but it's the shotgun I use almost exclusively for waterfowl hunting. The bottom eject helps keep rain & crap out of the action, the weight helps mitigate the recoil pounding of multiple shots with 3" & 3.5" steel loads, and when I swing through a bird it actually takes an effort to stop...