What size are your levels mayo? I have some that I think are around 20k wide and they work fine for the most part.
One thing we've noticed on a few occasions is there's a certain absolute x value where Box2d things stop working. To solve this, we take the instance on the timeline for that world and move it over to the left, lol.
What do you feel is the upside to the addChild vs. having them on the timeline? I know one advantage to having them on the timeline at all is that when you get levels as big as the ones we've got, you can work on them at 25% scale (on a guided layer so they don't publish at that scale) and tweak them. If you drilled into a library instance where it operates at it's 100%, you wouldn't be able to do that at all.
I'm in an all out war vs. creative trying to wrangle their intense visual stuff. Trying not to sweat it, but it's definitely on my shoulders and they just continually add detail. Guess that's why I'm up at 3am working on this stuff. Can't sleep when I know how slow some of the levels are running right now,