If you're thinking of the barrel nut as a switch barrel enabler, it's quite the opposite. It makes a switching barrels a bigger pain in the booty. Switch barrel should be done with a shouldered barrel on an action that's had the face trued and which is equipped with a pinned recoil lug or Barloc type setup. You don't torque them down. You go hand tight. When you use the barrel nut you need to put enough torque on the barrel nut to not let the barrel screw IN anymore which it'll try to do each time its fired (assuming right hand thread and right hand twist). Once you start shooting it it'll want to cinch the barrel in tighter so it walking out is almost a non-concern. You might check out the Barloc system by ARC. It helps with the pesky recoil lug problem. I'd still use the shouldered version and get your barrels set up with shoulders and eschew the barrel nut. That way headspace is set by the shoulder location and you don't have to carry gauges with you and you don't even need tools to switch barrels.
I've had mixed results with pre-fits and it really does come down to who you get them from. Some shops take great care, some don't. The most consistently fantastic pre-fits I've always gotten from Columbia River Arms (formerly Black Hole Weaponry). I'm running 5 of them right now on various match rifles. Fantastic stuff. They're polygonally rifled so they swage the bullet instead of engraving it. I get longer throat life since there's no sharp edges in the throat to erode, and on the rare occasions that I might clean one, they clean super fast and easy with just patches and solvent.
