Each artifact has its own entity file and uses a researchModifier to add the effect. For example, in RESEARCHSUBJECT_ARTIFACT_WEAPONBOOST you'll see the following:
Which basically says it increases damage by 12% (all percent values are in decimals in the files). If you wanted to change it to, say, 20% you'd just change the value to 0.200000.
Note that you need the text versions of all the files, which you can get by using one of the several ConvertData GUIs the modding community has made.