Extended Mags attachment adds 50% more pellets. Spas-12 8 x 1.5 = 12, Striker 6 x 1.5 = 9.
Range increases damage drop off 25% NOT the overall range. So really, range makes the pellets contain their highest damage, for a longer range.
Damage, well is the same as stopping power (1.4x or 40% more damage).
They balanced out the shotguns pretty well, though overall, that Striker setup is what makes it the best shotgun.
Edit: I can confirm this, no need to groan his post. Half of my kills are from shotguns, and I know what I'm talking about. Go in a private match yourself, choose a Spas (Or any shotgun that can have a Extended Mags attachment) and have a class with and without Extended Mags. Shoot at a wall, you'll see the difference, if you can count.