Hey Noz,
tenacity is just a niche. Even if you go for full tenacity build, the result will be damage dealed ~+5% and recieved ~-5%, while DH will give you around 7-10% damage increase.
Tenacity will NOT significantly reduce the heals needed, nor will it save you from a tank buster if you haven't saved a CD.
Tenacity is 'nice' for progression, but once you cleared a duty, it becomes more or less obsolete.
You should already be overgeared, your item level is more important than tenacity. If you die to tank busters, you haven't figured out your CD timing and/or your healers failed.
