I tank with my sorcerer pretty often and there are some problems with using dark deal like this. Also, I don't think heavy armor helps magicka sorc tanks. Here's why:

Your biggest resource as a magicka sorc tank is magicka, you don't really want to invest in stamina because your hardened ward scales off of maximum magicka (even spell damage doesn't help your ward), furthermore your ward has zero spell and physical resistance, meaning heavy armor shouldn't help you as you shouldn't be getting hit.

The way I magicka sorc tank is a bit like a ragdoll. I get tossed around a lot, allowing my ward to take the damage, and I only use my stamina to break free when I'm stunned. As for my taunt, I only use inner rage (or whatever it's magicka morph is called) since I maximize my magicka pool (to maximize my ward) and only use stamina to break free.

For these reasons, I wouldn't take that particular morph of dark deal, instead, I'd trade stamina for magicka since my ward and taunt require it. Besides that I wear light armor for the magicka regeneration and I try to DPS while I tank to add utility.

The big problem with "traditional" tanking with a heavy armored mag sorc is that Dark Deal has a cast time, so you can NOT block while using it.