My Bar-Sto .357SIG barrel for my 40 cal P99 is more accurate than the factory 40 barrel, but that is based on my handheld shooting at 15 and 25 yards. Not an accurate scientific test by any means. How much more accurate? I really do not know but the groups are "tighter". I cannot find my test targets.
For what it is worth, my Sig239 9MM is extremely accurate, and more accurate than my Sig239 in .357SIG caliber.
I am sure individual barrels and pistols will differ.
One characteristic of the Bar-Sto barrels is that they offer accuracy, but may be hard to fit to your pistol, in my experience.