The healers are healing regardless of if you take damage or not.
Threat is only an issue in the first few seconds of a fight. And even then it's not an issue really because of ToT and MD. And if those aren't enough then it's not gemming that is the issue, its a rotation issue.
Gemming the way you are suggesting means that you take less damage over the course of a fight. But you don't die slowly, you die in very short bursts of time. Crit/Avoidance (AKA RNG) can save your life sometimes, but you don't want to, and can't, rely on it because it is RNG and is not guaranteed to save your life whereas Stamina is always there.
Gemming the way you suggest is good for trash IMO. But even then I'd rather switch to a few DPS pieces.
And I really wish Tankspot would put an armory link thing in like EJ does! ^.^