There is, in my opinion, only three reasons to buy a barrel kit:
1: for a closed bolt marker to prevent the ball rolling out of the barrel, the etha is not closed bolt, not a lot of markers are nowadays, the most notable exceptions being most pump markers.
2: To chase high efficency, under boring can increase efficency, however the increase in efficency from an underbore is not huge and fairly irrelevent unless you are in a situation were your air supply is very limited (such as running a marker on 12 gram co2, or chasing 11 pods out of a bottle), also if humidity is high underboring leads to more barrel breaks.
3: Chasing accuracy, which in itself is a bit of a myth. There's a lot of people who will talk about the importance of bore matching your paint to the barrel. Actually studies have proven that matching paint to barrel exactly results in the lowest accuracy. The reason for this is that even very expensive paint isn't all exactly the same size. If you go after an exact match what really happens is some of your shots are a slight underbore, some are a slight overbore which results in greater fluctuations in velocity which is the key factor in accuracy.
The same study showed that overboring and considerable underboring resulted in the best, and approximately equal accuracy results.
TL
R don't bother, although anecdotally people attach importance to barrel sizes studies have shown no real improvements with them over reasonable stock barrels.