No problem. You are right I couldn't find anything that edits the different levels of tempering. The only thing I can think of is if you improve the weapons yourself and give them to the npc as your follower or you can make a copy of the weapon that the npc carries, change the name to "legendary weapon", improve the stats so it does legendary damage, don't use a temper object for it(so it can't be improved beyond legendary stats), and give the npc that weapon.