From my calculations I would still stack all res but try looking for armor with more armor to it. I have about 5.9K (maybe a little less) and about 530 res all and impunity is slightly better. These are just my personal calculations though and it could be different depending on the amount of all ress you have versus armor.
If you like playing games for fun & don't care for KDR then come join my clan. Where peps like u and me can hangout and have fun. http://hobos.comze.com
I'm going to be switching to Overpower I'm sure. Choice of 35% reflect or 10% Crit is better than minor resist/armor buffs imo. I'd have switched already but my Barb only has ~500 AR at the moment making me feel a little squishy in Molten, and for the purpose of speed farming, I'd still rather have a safety net whose dual purpose function is also a Fury generator.