I think its the reverse. But then again I'm playing as a Breton so I eat enemy casters. 25% spell resistance from the racial, 50% spell absorption from the Atronach stone and then the once a day power that basically gives you spell immunity for 60 seconds and massive magicka regeneration when hit by a spell.
Shoot, if you get 5 perks in the Alteration tree, you get another 30% magic resist passive, so as a Breton that's potentially 55% magic resistance with no active effects needed. Total anti-mage. I haven't made it to late game, but I bet there are a ton of casters at that point, and this would just steamroll them.