I'm an infrequent SC shooter, but I do a lot of low gun skeet. However, I've had very good SC shooters tell me they can tell the difference in lead when shells vary by as little as 100 fps. Maybe it's in their heads--entirely possible, since much of shotgun shooting is mental--but that's what they say.
And a difference of over a foot at 40 yards can be pretty significant. If you're shooting REAL birds as opposed to clay targets and you're poking at a quail, if you're underleading just a bit and the center of your fast load's pattern is just behind, you'll still likely kill the bird because of the spread. Back that center off another foot +, with the slow load, and you'll probably miss. Same scenario with a pheasant, it can make the difference between a kill with the fast load and a crippling hit on the rear end with the slow one, if you use the same amount of lead.
