If i want to do the same with a projectile that moves fast there is a clear visible distance between particle emissions.
There is no way to tell an sfx to emit at a higher frequently to compensate for the weapon's high velocity.
comparison of fast and slow projectiles:
Fast projectile:

slow projectile:

bigger version:
http://img19.imageshack.us/img19/9389/fasti.jpg
http://img62.imageshack.us/img62/2929/slow.jpg