Do you think that there is a worth while cost savings in relationship to the 'wait time'?
Form 1 for self-made or Form 4 for factory-registered SBR, there's still a wait.
My XCR is an early model, and I'm still running the old gas system. I had my mine cut and rethreaded approx 26 days after i got it. As far as I know, I was one of the first to go SBR (at least, only one or two others were actively posting about it on the various gunboards at the time). Because of this, there was little to no information about how they'd run.
The problem with having a barrel cut down is resizing the gas port. It was a trial-and-error method with me. I have a drill press and some knowledge of doing such work, so it became a matter of opening up the port, shooting, opening it up more, shooting again. Rinse, wash, repeat until I had consistent cycling. Add to that factor the several gas settings, and the fact that I was running suppressed.
My gas system and custom work is tailored to my individual XCR. It would not be optimal for every SBR out there. I did what I had to do to get it to run reliably with the M4-2000 that I have dedicated to it, and run reliably without. YMMV.