I have had a butt load of custom 223's, 223 AI's, and 22/250's & AI's. I have used both the 14 & 12 T, I can not tell the difference in accuracy as all of them shot in the very low 2's with occasional 1's when the conditions were right.
For benchrest application, a 14T only.
Advantage of the 12 twist for a varmint hunter is that is better stabilizes the Sierra 55g Blitz king, 53g V max, 60g V max, and in the 22/250 AI with 12T, I blew up coyotes with the 60g berger match at 3700 fps using Win 760...shot bug holes.
For giggles and grins, I did build one 223 with a zero freebore with a 16T that I got on a deal. I shot the 40's at ungodly speeds along with the 50g Sierra Blitz...p. dog, ground squirrel, jack rabbit rifle.
Barrel time= time a bullet spends in the barrel. Barrel time has some relation to how pressure builds, and just how barrel time relates to pressure is voo doo magic for me. Could be the barrel ID has more to do with pressure than twist rate.
IN the 243 AI's that I have had built, the 10T sure built pressure faster than the 12 Twist, same barrel maker, same reamers, same throat lengths...again...many factors.
For this reason, I buy barrels for the bullet weight that I intend to shoot, NOT get the fastest twist per the caliber. Another huge issue is the longer throat that is needed for heavier bullets. A guy could start out with minimal freebore, then as the leade gets some long wear, then go to a heavier bullet in a fast twist.
I like to shoot very, very tiny groups, then when I can only see a p. dog's head sticking up out of a hole at 300 yards, I can suck him out of that hole!