What he said! If you modify "any ol' weapon" that all NPC's have access to, then all of 'em are gonna have the same stats and they will crush you!
Pick only weapons that have a "unique tag"... that is only one is in the game at a time. The same goes for armor. Users of the Dragon Age Toolset would be familiar with the concept of a "tag" which is the under the hood identifier for a particular item in the game. Common weapons of the same name and type all have the same tag and will all be modified by an editor. So again... you should only modify those that appear only one at a time in the game and you or your companions have it!
I don't make it really high (only change the base damage of swords to 27 or 28 making tier 7 swords have like 40 damage). Instead of relying solely on the weapon to kill, I raise the strength attribute of my characters. The Blightblood sword with a base damage of 27 has a max damage of 43.20. With a strength moddifier of 63, the max damage is 125.1 before you add in critical hits\back stabs or talents. Enemies will never has massive strength like that.
You can also increase your dexterity to reduce the chance of being hit and increase you constitution to increase your health. Remember, in an RPG like Dragon Age, there are many factors that determine combat.