Me and my shootin buds did some chronographing with 22 LR ammo and we found that the most efficient bbl length is 10". What I don't know is if this info is a good indicator of performance across the board, or just in our guns.
CCI Stingers
980 FPS (FA Mini-Revo 1-1/8")
1192 FPS (Ruger Bearcat 4")
1516 FPS (Ruger MK II 10")
1550 FPS (Ruger 10/22 18")
CCI Velocitors
1126 FPS/ 34 AD/ 165 ES (Ruger 10/22 18")
As for the stopping power or lack thereof, I think that it's more how you wield it than the actual caliber (perhaps in most cases at least). In a raging firefight of course I would want a larger caliber that has more penetration, but for most of us, a 22 would be enough to make most evildoers back down. I'm looking real hard at Rugers Charger pistol right now. I think it'd make a dandy house gun with a red dot and a fat Mag.