I see you can get two part barrels: Instinct says that they would not be as efficient as a one piece?
It is a physics conundrum
However (forgetting the specific science):
The first x inches are key in Paintball’s journey
This is where you decide on precise/over/under bore in relation to what is optimum for your needs
Once beyond y inches the ball no longer requires acceleration, and now needs to think about the moment it leaves the barrel and smacks into the open atmosphere
This is where the porting should begin, ease the pressure from behind it and ‘equalise’ with the atmosphere
Finally it pops out of the barrel
Anything beyond the key x area is pretty much wasted effort, so bore sized barrel kits concentrate on the back
You could have an optimum one piece barrel for each situation (which is probably cheaper then a barrel kit)