I ordered a 5.5 barrel (the slab-sided one - the bull is too heavy for me) and wanted to share some observations.
The overall fit and finish are somewhat better than on the 4" barrel that shipped with the gun. The breech face on the 5.5 is flat (the 4 looked like it had been machined with a scout knife), and the feed ramp has been polished (machining lines are still visible, but at least they made an effort). The breech face has still been painted all over (this appears to be done to hide the usually poor machining), but the polishing to the feed ramp was done afterwards, so it's not painted like the ramp on the 4" was! There is that damnable thick paint 1/2 inch down the muzzle, totally filling the grooves and making the initial cleaning a nightmare, with paint shavings and a scary scraping feeling on the cleaning rod.
The Hi-Viz front sight is much narrower than the stock sight, and it may be removable. The round Hi-Viz dot might present a sighting problem as-is, without anything on the rear leaf to line it up with. I'll follow up on this post when I get to the range. I'll post some pics too.
First I put 10 rounds through it fast to clear the paint from the barrel. Damn their finishing department.
I put 500 rounds through it yesterday. It ejects forward about 5 feet, in contrast with the extreme throw of the 4" barrels. Accuracy is notably better, but I'm going to attribute that to the fact that the hi-viz sight is much narrower than the plain sight. The hi-viz sight is shorter than the plain sight, so the elevation needs to be screwed down pretty far.
Target acquisition is vastly faster, so shooting steel is more fun (hit the 8" plate at 100 yards a few times), but I don't think the hi-viz is any use on paper at all. So, a big thumbs-up on this barrel for plinking, but for bullseye I'm going to have to come up with some kind of mod.
Oh yeah, almost forgot... The ramp came unscrewed from the barrel while I was shooting. Don't know why S&W can't seem to get that right. A little Loctite and all was well.