I have one of the 21" turkey barrels for my 870 that is threaded. My brother has one of the 20" rifle sighted barrels which is also threaded. I have found them to be the most useful overall barrels I own. Not the best choice for clay shooting or pass shooting at waterfowl by a long shot, but useable with the right tubes and some practice.
They are perfect for SD, turkey hunting, deer hunting, close range quail, bear defense, or as a general purpose survival gun. The longer barrels are MUCH better for overall hunting/clay shooting, but useless for many other tasks. The 18" barrel you have now is quite good as a SD weapon, but basically useless for anything else.
Overall I like the idea, but think something 2-3" longer would be a bit better. Since it is a Remington barrel, I'd stay with Remington tubes. I could see Beretta tubes in a Remington barrel causing serious confusion down the road.