Technically, the LX600 uses 600 series inks, and the LX850 runs 610 series inks, so they are not the same, but you should see similar performance.
I don't know how tight your heavy knit material is, but you will want to use the ink collector kit regardless of how tight you think it is. There are plenty of fabric media settings that should work out, the printing performance will depend on how well the supplier spooled the media. If it is loosely spooled, or was not spooled with enough tension to pull the stretch out of the material, you will have a tough time holding registration through your roll of material.
With fabric, it's all about how well the material comes off the supply roll. If the edges of the roll are straight, and the material does not have a bunch of stretch as it feeds, you will love it. Anything else and YMMV greatly.