LS long slide is 5.25" barrel, regular is 4.25, compacts & Mill Pro's are 3.25"
Longer barrel will give slight velocity increase. More weight out front for less muzzle flip, more weight for less felt recoil, and longer sight radius makes more accurate sight alignment easiers, but I don't think it won't shoot tighter groups just because the barrels longer.
3" group is outstanding, gun even in a machine rest probably can't do much better. Groups are measured by the largest circle that would contain all shoots, generally only 5 or so shots, often though we just measure the distance between the two farthest away, guys that get real picky then subtract the bullet diameter to get center to center. Since you called it a spread just curious what you meant.
My Mill Pro shot low at first but came up as I got 4-500 rnds thru it, been great for 2500+ more since then.