The knockback is the case of some bugs at least in the previous version, where you were able to negate Cyberdemon's damage fully using Fireangel. But that was a known bug and it is strange that it isn't fixed already.
Another thing that might be the case of the problem is that knockback calculates after every shot out of your 5, and thus damage dropoff is different for every of your 5 shots. And as far as I know, firestorm modded DBS is a bug, so if you do this, "use it at your own risk".
And I don't think that the fact that you can't one-shoot caco's makes them be that bad. Shotguns are meant to be crowd-controllers, so if you do enough damage to nearly kill one caco, you'll deal the same damage to every caco in your shot radius, which is already big enough to out-perform most weapons in the same situation.