I had a pair of .45s a long time ago, one a 11.5" with extra-length flash-hider, and a 16" The velocity of the 16" was not less than that of the 11.5", but it wasn't enough more to bother over. Now, that was with factory ammo. Reloading, and using slower-burnign powders, you could probably get a velocity increase, but at what cost? You'd simply be pounding the carbine for a few more fps.
9mm is different. It starts at enough-higher pressure that you get a sustained push through the full 16" of barrel. A 16" is markedly faster than a 11.5", all other things being equal.
However, not all things are equal. Some barrels are faster than average, and some slower. If you happen to have a "fast" 11.5" and a "slow" 16" you may not have any difference in velocities.
In the end, do what makes you feel good.