Theoretically you'll be more accurate and have a higher velocity with a threaded barrel, but in practical terms, the difference would likely be negligible. I haven't seen actual numbers before, but that's probably because the difference is so small as to really not matter. If you're doing it for looks or because you plan to buy a comp or suppressor, then it's certainly worth it. But for accuracy alone, I really doubt it's worth the money.
As for your other questions, I don't have experience with any of that, so I can't really help you.
