I'm building a magic item to sit on a cathedral tower. Very basic question, but am I correct that on triggerring the spell effect it ignores local aura, so I don't have a -15 penalty (except to penetration, which is irrelevant here -- it's a Ward versus Lightning in a statue?) and the effect works as is in any other aura?
I don't believe aura directly affects penetration at all. It affects it indirectly, by lowering or raising your casting total, which doesn't come up in this case, because magic items don't have a casting roll. Now, some think it would be fun to have the aura affect the magic item, but I think that's a bad idea. The way I think of it, the magic item is like you casting the spell in your lab, under controlled conditions. Now, if you (instead of making an item) just cast a spell in your lab and then went to another aura, would that affect the casting total of the spell? Would your spell suddenly not have a high enough total and "Pop"? I think we all agree that would be a huge mess....
You are correct. If the item doesn't need to Penetrate to have its effect, then the local aura is irrelevant.
The local aura will affect the Magic Resistance of things standing in it.
The local aura might also affect the Penetration. The RAW say that Penetration is affected by aura, and the RAW explicitly says that for magi this is already taken into account by the aura modifier/penalty to Casting Score roll (i.e. you don't get penalised twice). However, the RAW goes on to say that for creature powers, where the Penetration is not based on an already aura penalised roll, the Penetration is modified directly. I would read this as suggesting that for items (where the Penetration is likewise not based on a aura penalised roll) that the Penetration should be directly modified by the aura.