I haven't been able to find any details on this... Basic setup: I found an artifact with a ridiculous amount of +'s, whose item type is a "Filigreed Silver Shield". Meaning, I could upgrade it to a Mirror Shield. If I do so, will I lose the artifact bonuses or will they be retained?
Yes, sometimes the artifact bonuses are not enough to offset the upgrade advantages. You have to look at things on a case by case basis.
The character is running "the art of unarmed" and "shield-bearer" and dual wielding shields. I was using two "filigreed silver shields", and rotating the clockwork defender or caketown shield when appropriate. The shield in question has the most +'s on I've seen on any item. +3 , +2 , +1 , +1 , +1 , +1 , +1 , +1 , +1 , on top of the normal stats. My vote is, the extra 24 isn't worth giving up all that.