As everyone else has said, freak bored barrel is the way to go. A phantom barrel is amazingly accurate stock, once you bore it for freak inserts, it gets even better. No new barrel needed.
You don't really need a stab for using HPA, that's more of a CO2 thing. HPA is pretty darn consistent as is.
Another thing is to be sure you keep the gun squeaky clean. Phantoms can run really dirty, but for best consistency and efficiency, you want really clean internals. This allow the hammer to hit the powertube the same way each time.