I actually use a 18.5" M1 for 3gun and like it. It's very light and fast in the hands. Velocity isn't a problem because shot shells peak pretty quickly. I do have interchangeable chokes though, and I think if you get targets much farther than 18 yds and smallish you probably will have problems. I've never looked into threading a barrel, so I can't help there.
I did the standard set of mods to mine. Got rid of the pistol grip stock, ported for quads, extended chokes, new Airtech Limbsaver, bigger bolt release, fibered up the front ghost ring sight, +5 Nordic (thinking of going longer).