I would post this over on 300BLKTalk where Robert Silvers from AAC is and see what his thoughts.
I dont think you would see any kind of appreciable increase in velocity personally. Maybe 50fps. Its not going to turn the gun into a real .308 velocity gun with the extremely small powder load.
I had a buddy that wanted to get a 20" 300BLK Barrel, but after research and question asking it was determined that the extra 4" of barrel wouldnt really do anything for velocity except maybe a VERY small change since the 16" barrel is even overkill from a powder burn perspective.
I dont think I would go any slower than 12 twist personally. Black Hole Weaponry has an 11 twist and they are polygonal rifling which the polygonal SHOULD help increase velocity slightly too.