The 20" Katahdin barrel is a standard factory offering, not a special-order barrel. I have two, actually, a muzzleloader and a .45-70.
As far as accuracy goes, mine has been fussy. Treat it right and feed it what it wants, and it will shoot one-hole groups at 50 yards, and under 2" at 100. The best sabot/pellet combinations seem to produce right around 1.5-1.75" at 50 yards, and around 3" at 100. It only gives up about 150-fps from a 26-28" barrel and is quite a bit handier, IMO. Even with a 3-pellet charge, it will break 2000-2100-fps with a 250gr sabot.
The .45-70 is every bit as accurate, if a bit harder kicking IMO. The biggest issue with the .45-70 is that the ProHunter forend is pretty thin and cheap and doesn't seem to want to hold up to the recoil is you hold the forend. I'm on my second forend, and afraid it has already started to warp.
And no, mine is NOT scoped!