It can get confusing because there are two different kinds of Pro models. One is the long slide version with sights set up for target shooting and competition. The other is the same as the regular full-size M&P but it comes with night sights and a slightly better trigger (still has a terrible reset, though).
And a longer barrel won't directly increase accuracy. In fact, a longer barrel will often decrease accuracy, especially in rifles. The bullet is actually stabilized very quickly and a longer barrell isn't going to stabilize it any more. Instead, a longer barrel is going to flex more, causing a slight decrease in accuracy. Sniper rifles have longer barrels to allow more powder to burn and therefore get more range out of a given round, not to increase accuracy.
However, with open sights many people will be more accurate with a longer barrel because the sights are farther apart (sight radius), allowing the shooter to aim with more precision. Also, a longer barrel usually gives you more muzzle velocity because more powder is burning before the bullet leaves the barrel.
0331: "Accuracy by volume."