I'll be a little more clear. The Arrowstorm is fairly rare but I don't think that it sees a lot of play because of the scarcity. I've never had one but supposedly it can be a force. Nice find. In short it's a multi shot springer, what's not to love about that?
Granted for organized play the ideal situation is to have everything standardized and I like the class system that you proposed. Equipment wise everyone would be on an even playing field and true skill in combat would prove to be the deciding factor. There are of course things to overcome with this, for example: are we organized enough to pull this kind of thing off?
Frankly I don't think that there's a wide enough audience for this to happen but I am quite isolated where I am. They do have regulations for dodgeball after all so there may be hope yet.
Back to your question though. Why bother modding a toy that you can't use in a war? I look at it as an engineering challenge. If you regulate what people can and can't do on the modification front people loose their creativity a little. Who was the first one to mod Nerf? What went through their mind?
I'd like to think that it was to squeeze the most power out of the gun as possible. Pushing the given design to the breaking point can be greatly rewarding. The process itself may be its own end for some, not what you ultimately do with it.
Of course within your constraints of organized play there could always be an exhibition class. This would cover things like distance and accuracy shooting of non human targets.