Doesn't matter how well you gear your tank companion, he's always gonna take around 20% more damage than a player tank (and much, much more if you factor in positioning+cooldowns).
Also notice I didn't say it couldn't be done, what I said is that it isn't fun and can become very frustrating/inefficient. I personally don't find over-complicating things and being a healbot to my AI pet fun, to each their own.
As for Heavy Armor making a difference, its roughly 10% less damage, which is deffinitly worth pointing out.