Stock barrel is pretty damn good, and I'll tell you why.
The honing is great, the length is nice, and it's got a good amount of porting for a pretty decently quiet shot compared to other barrels. The stock barrel also allows you to shoot not-so-great paint because of the large bore. Now, when it comes to shooting good paint in an over-bored barrel, there will be a slight change in consistency perhaps and yes a bit of accuracy. But buying a new barrel for accuracy really isn't the best bang for your buck.
Paint makes a much greater difference in accuracy than the barrel, given the barrel is clean and is large enough for the ball to get to 300fps.
So, instead of buying a new barrel that will cost you at least $40, spend $10-$20 more per case and get great paint, the many varieties of which you can find reviews for on this site.
Sources: shooting dye cg using stock barrel, freak barrel with 12 inch teardrop tip, and freak barrel with 16 inch AA tip. Ended up using the stock barrel because apparent accuracy (the accuracy I perceived without having analyzed it in a controlled environment), didn't really change, but the over bored barrel was nicer to the more delicate paint. I also ended up borrowing some of my friends paint and was very happy I had the stock barrel fitted.
Shaft 4 back in 689 or 685 since stock barrel is not a shaft 4 and a decent barrel it also has a huge barrel size of 692/3 don't remember which one. And as stated above by better grade paint and you're aces.
I have a Freak kit that I use to good effect on all my markers, Etha included. Since we go through sudden temperature changes frequently here, even within the same day I like to be able to change the bore size on the fly. But as said above the stock barrel is pretty good and you will only get marginal improvements if you change it. Good paint is the key.
Again as said above you would simply pick a CP barrel bored at 0.685. In between that and the stock barrel at 0.693 you should cover most situations. If you're willing to spend a bit more get the PE shaft kit with the 0.685 and 0.689 backs. You might as well get the Apex compatible front and an Apex tip and have some fun with that.
A boost kit is an awesome choice as the shaft 4 are some of the best barrels on the market. But if your looking for lighter weight, look into the deadly winds barrels, its nice to have so many different bore options. Not to mention its like not having a barrel at all on it.