I never had a fit problem. And, no. I had no issues with a break in period. Some say, if your going with a conversion barrel you might want to change the extractor, ejector, and maybe a spring or two. I used a .40-9 barrel in a Glock 23 without having to change anything.
Lone wolf is the least expensive of the aftermarket barrel manufacturers, but I was pleased with the results. I had a EFK barrel that jammed up a lot.
I have experience with 2 LW barrels on G21 - both dropped in without any problems. One is standard length was used with original slide, the other one 5.15" is currently (ab)used with LW long side. Both were not ported. Ran about 1K rounds through each, no problems detected.
I have a .40 conversion for my G29, it works pretty good for a cheap drop in caliber change. The only thing I will say is the .40 cases basically dribble out of the gun, I assume it is because it still has a 10mm recoil spring. But all in all, not a bad investment.