Firstly, in regards to you own stats, drop the Accuracy and get your shield and Absorb to 50% each, thats far more important then Defence or Stam to start with, and threat is a non-issue, especially with how taunt works in SWTOR. Aim for 20% Def, 50% Shield/Absorb. Once you hit those numbers, aim for 27% Defence with a stim active, this will give you 30% Defence during combat when using Retaliation.
Once you hit 27% Def, 50% Shield/Absorb, my initial thinking was to start stacking Stam to help soak up any non-avoidable/mitigated dmg (ie Force, Tech, Internal, Elemental dmg), but thinking back over it, im looking at pushing my Shield and Absorb beyond the 50% marks instead of the extra Stam due to how fights currently work (nightmare mode may change this).
Currently most/all the "scary" parts of each encounter when looking at EC HM can be avoided/mitigated by Defence and Shielding, and reducing those dmg periods is going to be of more use to you and your healers then having more health, but getting hit harder. The parts of the encounters im talking about are things like Zorn's Berserk phase, Tanking Kephess himself, etc
So, essentially, any dmg you can avoid/mitigate is basically extra health, so your getting the best of both worlds, while going for pure Stam means, yes you have more health, but your getting hit harder, so its not really that useful.
Ideally on your characters stat sheet, you can hit in BIS gear around 27% Defence, 53% Shield, 55% Absorb.
(Big thanks to Aoroc from Cali Killed Nox for helping me with alot of this info)