I would certainly not call the Spencer anywhere near a 300 yd cartridge. By that I mean you might accidentally hit a large (buffalo-size) target at 300 yds, and probably seriously injure or kill something out that far (danger space), but realistically, 150 yds is probably the maximum practical limit of either the .56-50 or .56-56. Even a replica chambered in .44-40 is probably best limited to 150 yds or less. Compared with the standard infantry weapon of the Civil War, the .58 caliber muzzleloader, which was capable of maybe three rounds a minute under ideal conditions, by an experienced troop, and which could reliably kill out to 300+ yds, the Spencer was a Civil War a$$-ult rifle, intended to provide relatively high firepower at close range. The same was true of the Henry Repeating Rifle, except the Spencer was harder-hitting, though of more limited magazine capacity.
Yes, the Spencer windage is not adjustable, except by drifting the sights.