Had I known that I would not have made note of that thanks for the correction.
As for a weapon dealing its base damage plus twenty extra damage that would kinda be the same as a weapon dealing its base damage plus twenty extra base damage.
the base damage plus the twenty extra damage is still base damage plus twenty extra damage if you think about it. Even if the base damage increased that would still be the base damage plus twenty extra damage.
No like I mentioned above it's very different - 20 extra damage is like a 20 damage fire enchantment, this extra damage won't get any stronger if you have 5/5 armsman or +160% fortify one-handed enchantments, only the
8 base damage will. If however it is 20 extra
base damage it means that the weapon will have
28 base damage against undead instead of the normal 8 base damage - to put it into perspective that is the equivalent of an unsmithed Dragonbone Warhammer with equivalent perks, or in other words it is roughly (not exactly since apparently increased weapon skill doesn't affect the extra damage) as if the silver sword turns into a 3rd rank Legendary Dragonbone Sword against undead foes. Does that show how much potential it can have? It can even still be enchanted just like normal too. For players that don't rely solely on smithing Silver weapons fill a very unique and powerful niche if they get +20
base damage vs undead/werewolves etc.