Have you tried using the same technique as the upgrade.ltx files? When upgrading weapons, the file dosent change the value but rather add/subtract from it and it looks something like this fire_dispersion_base = -0.07 ;0.7 Notice the minus, so perhaps adding lines like
max_weight = +5 max_item_mass = +5 max_walk_weight = +5
Ive also opened the outfit.ltx file and looked at how they upgrade the weight and they use a variable called
additional_inventory_weight = 0 ; +max_walk_weight additional_inventory_weight2 = 0 ; +40 kg
This is taken from the bandit suit or something, try adding this to the artifact file. By comments it looks like _weight is max_walk_weight and _weight2 is max_weight or max_item_mass.
Give these 2 theories a go and let us know. Its a good idea.