I find very fine husk particles make it through even a well set grain bed. They collect at the bottom of my "grant" (bucket the tube from lauter tun goes into) so I can carefully run the clear wort off into the kettle leaving them behind. If your mesh bag is fine enough it might exclude them. Other substances are fine and light enough they might squeeze through. But they are also most effectively left as "top dough" on recirculation and excluded from a lautered wort. Again, I don't know how effectively they might be filtered by a bag. (Thise "malt pipes" would worry me.) You can always sample your wort in a glass. If it's clear, good. If it's at all cloudy, not so good.