well overload does ~8-12k dmg before spell resists while beeing buffed so without some bug (wich the game contains way to much sadly) the dmg you observed is not possible.
I'm not sure about Overload attacks, since I don't play a Sorc and I have no clue how hard it is to use the attacks etc. But I can say with 100% honesty, that I have been hit with 18k to 22k overload light attacks by Sorcs. I know, because I stared at the death recap and scratched my head. I knew it was something to do with an ult though, so I just shrugged... respawned and went back to feed some ap... err.. try to get ap. :P
And that's with 13k magic resist when I was nailed by those. Not a ton I know, but not naked either. Obviously, I should have been rolling.
Well these hits are possible if you invest a lot in spell damage and magicka, more than I would call a healthy balance.
And 13k spell resist is not only not a ton, it's easily completely ignored.