With the increased threat gen from your tank buff, the Willpower you get on tank gear is more than sufficient for holding threat. If you're not holding threat, you're either undergeared compared to your DPS, or you're new at tanking and don't really know what you're doing.
Damage should never be a focus over survivability, no matter what level of gear your at. As said, the easier you are to heal - the easier it is on your healer and the more they can focus on other members / CCing. While no, this isn't WotLK WoW where "Stamina is king", a healthy padding of health just makes things a lot easier on your healer and thus easier for your group.
The only thing that you could, once sufficiently geared, swap out for more Willpower (if for some ungodly reason you need more threat...) is the Mod. Armoring should always be a tank piece, and your Enhancement should always be a tank piece. The Mod could be switched out if you feel you need it, but the instances in which you do will be rare. Also, there will be new Operations of a higher tier (at some point). So you could be comfortable tanking the current ones with slightly less Effective Health, but I'd bet the new Ops will raise the bar on difficulty and you'd want that survivability instead of DPS.