*The discussion of the creation, fabrication, or modification of airgun moderators is prohibited. The discussion of any "adapters" used to convert an airgun moderator to a firearm silencer will result in immediate termination of the account.*
If you already have a Maverick and you don’t mind changing your setting a little then I would grab a larger bottle and barrel kit and save yourself some money; unless you want two rifles.
I have a 25 Mav Sniper 700mm and a 500mm barrel also. If you want more fps at lower reg/hammer settings then by all means get 600mm barrel and a 580cc bottle. I would not buy another Mav in 600 if I were you, it is really easy and fast to swap bottle and barrels, like under 2 minutes. As far as having to adjust your reg or HS that will depend on your cal and what your are wanting to do with it, so flexible.
I have a compact .22 Wildcat MK3 BT (maverick with a different stock), that I put a 600mm barrel and 580cc bottle on to shoot slugs. It's my favorite setup now. So quick to change the bottle and barrel out. No need to purchase another gun.
If you mainly use the gun for pellets, or light slugs, I personally see no point of making the gun longer. The 300 cc bottle, and 500mm barrel already gives plenty of shots.