I was using almost the same spec and 3 piece frost resist jsut sitting at 540 def, i had no problem wih agro yet we weren't doing hard mode (25man).
I never had to use cool down with 3 piece of frost resist. in 10man were that 2 or 3 healers? I didn't need to use cool down at all in 10man wiht 2 healeres (3 piece of resist gear), nor any healer cd were needed (hardmode). I am thinking if your healers can't heal through it with 3 piece of resist gear they'll need to be better.
of course you need to avoid dmgs like biting cold and icicles, if you get killed by these it's your fault
i will be using no resist gear for more threat if we are doing hard mode 25man in future
I also use intervene during flash freezes. you might want to try glyph of vigilance