The difference won't be nearly that big. QuickLOAD offered up 4½% velocity difference for a very fast powder and 6½% for a very slow powder in this instance. The reason is added barrel length won't stop increasing bullet velocity until the barrel is so long that the pressure drops to where it provides only enough force to overcome bore friction and no more. Even for the .22 Long Rifle, which uses very fast powder and has a huge expansion ratio, velocity won't stop increasing with barrel length until you get a barrel 16-20 inches long. Normal pistol barrels are nowhere near long enough for that to happen, even with fast powder loads.
Unclenick, that's not too surprising as far as velocity increase with barrel length increase, but when you examine load data, the medium/slow burners give the highest velocities to begin with. So the 6.5% increase from additional barrel length further separates even more from a faster burning powder where 4.5% is the velocity increase, even if proportionately considering the difference. Like say a medium slow burner giving 1200 FPS in the short barrel but 1278 FPS with a 6.5% increase. Compare that to a faster burning powder load at 1150 FPS from the short barrel where with the 4.5% increase for the longer barrel you get 1202 FPS. That's a 6.3% improvement by using the slower powder vs. the faster powder and I'd consider a 72 FPS improvement for something like a 115 gr. JHP in 9mm to be worthwhile.