Pretty sure it's high enough that you wont be able to hit the cap very easily and sometimes not at all.
Also for merc, I think the merc's aura's stack also, but they do not stack with your's, it is separate.
Also if the merc is act2 and comes with defiance i'm pretty sure if he had defiance items they would not stack.
So items stack but not with an aura you have on a character as a skill or for one that comes with a merc.
If you wore holy shock items, you'd have no reason for holy shock as your main aura.
And your merc wearing holy shock gear wouldnt make much sense either.
I feel like I explained this terribly but hopefully you understand what I'm saying.
And hopefully I didn't give any incorrect information lol.