It would be pretty nice to have a one piece inner/outer barrel but the questions are: How would you install a hop up? and where would you put it?
With a little thinking and drawing out of blue prints, it's not all that impossible. Especially if you know how to index your threads (Make what you're screwing on stop within less than a thousands of a inch [.0001] of a certain mark) of the barrel with the receiver. I've had this planned for a little over a year, and had the chance to mess around with small models about 6 inches in length, just never had the money for materials to make a full rifle when I had access to the machinery, and now don't have access to the machinery now that I have the money.
From what I can gather, if done properly (would be far from cheap), and with a fixed hop up for a set speed/weight it would be more accurate. As for how much, don't know. The downside to it though, you would not be able to work on the gun yourself, but would have to send it back to the builder.
I keep telling myself I'm going to finally build it someday...