I have a Bushmaster 24" Heavy-Barrel (Pre-ban config) that came with an A2 birdcage + crush washer.

Is it threaded 1/2" x 28 TPI?

I was thinking about getting a DPMS Levang Linear Compensator - how well does it work? Does it affect the POI or trajectory of the bullet? Do I need a crush washer or peel washer?

I've got a Levang Linear Comp on a mini-14.  It is a fine unit.  Symmetrical about the bore axis, so positioning with washers isn't required.  Accuracy with the mini is 1.2 inches at 100m, so it does not adversely affect accuracy.  The beauty of it is the blast gets ported forward, not back to the shooter.  I'm very happy with mine and would recommend it to anyone that had a threaded barrel to protect.

Does it increase recoil? What about affecting the sound of the blast?

The beauty of it is the blast gets ported forward, not back to the shooter.  As far as recoil, that is hard to measure in a gas operated .223, not having much to begin with.