2 Questions

[Solomoriah]

The rules say "double damage" without further details for a reason. The GM must decide what it means.

For the record, I double the full damage including bonuses.

[orobouros]

With these items is it fair to say that a thief can backstab with impunity assuming he doesn't fail his success rolls? These items seem quite powerful.

I would still require an attack roll. One, this keeps the player guessing if they were actually successful. Two, unless an enemy is truly incapacitated, I think there's always a chance of failure for trying to attack (magic spells that automatically succeed, like magic missile, aside). Full plate mail is going to be hard to attack from any angle, so a highly armored foe will still get some benefit. Either allow anything but a 1 to succeed, or give a very generous attack bonus of, say, +10. An attack bonus can also vary based on situation. A loud battle going on? Maybe it's more like +12. It's deadly silent and the foe knows a nearly invisible enemy is trying to attack? Might really be more of a +4.