Some rounds, such as a 45 ACP, don't benefit much from longer than pistol-length barrels. Others, such as 9mm+P, benefit greatly. So, it all depends.
I've worked with the 44 Mag round a lot. While I can't quote velocity figures off the top of my head, I can state matter of factly that it does benefit greatly from carbine length barrels (over pistol length barrels). I'd subjectively guess the optimum barrel length for fully loaded 44 Mag rounds to be in the 18 - 20 inch length.
Keep in mind that pistol calibers generally use faster burning powders than cartridges intended for rifle length barrels. But if a lot of that powder is burned outside the barrel (i.e. muzzle flash), then it could benefit from a longer barrel.
B. Noz.