Yeah, you'll always get more velocity out of a longer barrel. More velocity equals more energy, which will almost always be better (as long as you don't grossly over-drive a bullet's design). A 6" .45ACP barrel would probably gain about 100fps (+/- 30fps) over the 4.6" barrel of a stock G21. The longer the barrel, the more time the bullet is in the barrel, and the longer the expanding gasses from the gunpowder have to do work on the bullet. All of that equals more velocity. If you're hunting with a pistol, conceal-ability concerns go out the window, and a 6" barrel is a great idea. The +P loads will be a little bit faster than the standard .45ACP, so that will help some, too. I'm sure Bar-Sto or someone COULD make a custom 8" G20 barrel. It would likely be expensive and could cause cycling issues. You would get some SERIOUS velocity from an 8" 10mm barrel, though. Lone Wolf makes a threaded 6.6" barrel for the G20, I think.