Some guys did velocity testing with different barrel lengths for different cartridges by cutting a barrel down one inch at a time. Here's a link to their results for .45 ACP. In most bullet weights, going from 5 to 16 inches should get another 100 feet per second, which is significant for .45 ACP, which runs 1000 to 1100 fps for most loads. On the other hand, the normally subsonic .45 bullet will be pushed supersonic, so it will probably be quite a bit louder coming out of a longer barrel. Note that muzzle velocity actually drops going from 16 to 17 inches.