I've asked this of some of my friends also. They say that while booster doesn't affect the shot speed (6 hits per second I think), it does affect the start up time. There is a slight lag between when u press the hurricane button and when the first shot is fired. My friends tell me that this small lag time is what gets decreased by booster and speed infusion. You might notice this more in a boss battle where the boss can magic attack you (like pap) and you have to frequently restart hurricane.