The oem trigger can be modified several ways to get big improvements, and one mod duplicates what the GRT does, so if you're into doing it yourself I have the info, write me at chevota@hotmail, remind me of who you are and I'll send it to you. I also have info on tuning the whole gun for better power, accuracy, smoothness, lighter cocking, quieter, and to be less abusive to the scope.
As Race said about the hold, it's true you don't need to use the artillery hold. I don't use it either, what's important is the hold is the same for each shot and the artillery is simply the easiest to do, especially for noobs. On top of that each gun is different in that some are more hold sensitive than others. A tune will reduce the sensitivity as well so one more reason to do it.
The reason for the hold is because unlike other guns or firearms the springer gun moves around before the pellet leaves the barrel, so if you hold the gun differently the gun moves differently so it'll be in a different position when the pellet comes out. Forend grip, shoulder position and pressure are all factors.