Hi Phi,
It's always interesting (more or less) to know how things work. However, as some of the others have implied, what would/could you do with the information even if there was a definite answer. IMO, shotstring is not something that can be used to the shooter's advantage and also not something to worry about. It exists but if you shoot behind the target, none of the shot will hit it and if you shoot in front of the target, the shot string goes by so much faster than the target is moving, it will not help you there, either. Shoot the best, hardest, roundest shot you can afford and keep swinging it, like John Anderson says.
John Anderson offers excellent advice as always, but in relation to what you've said, I have to respectfully disagree with you. The dynamics of a shotstring change when a swing is applied to the discharge (or so I've been told and I've read). The shot is moving a great deal quicker than the target and if the pattern of a shotgun swinging is wider, taller, thinner than that of the pattern of a shotgun that is simply being pointed and fired like a rifle, then you can certainly play that curve to your advantage.
