Thats not how tencity works, iirc having 2500 tenacity only equals like 6% mitigation, some has the maths in the tank gear thread.
Onto the actual thread topic: If you're using your toolset properly there is 0 need for tenacity melds at all, the amount the game gives you is more than enough even when avoiding it, as you can survive everything without melds pretty handily, even in savage currently while under geared. Your healer won't notice a sizeable difference, so its better off dumped in an offence stat. Tenacities healing also only effects your own healing, not healing recieived