I think I have some info on this, as I have been playing with the chrono yesterday!
Since My 9mm Edge works best with long rounds and my buddy shoots short ammo in his CZ, I thought I should use some of his rounds in my gun and the difference was quite noticeable.
Using 4,0 grain of N320 with 124grain Berry's bullets, I got this:
COL 27,7mm: average speed=1037fps
COL 29,75mm: average speed=970 fps
So I guess it's safe to say that a longer COL leads to more room in the case and lower pressures, thus lower speeds.
BTW, I achive higher velocities with N340, but I like the snappy feel of N320, although it's harder to make minor (yes, that sounds funny), while staying within Vihta's loading tables.