The full T2 bonus for priests is “Your Greater Heals now have a heal over time component equivalent to a rank 5 renew”, this is hugely overvalued in the calculation. Currently it shows as being about 45% better than anything else in game (or Naxx).
While its not a bad set bonus its not that good. Its also not practical to give you full benefit off each cast as its a dot applied after a heal. The specifics aside the upgrade and BiS features become useless for armor when the 8 set is applied. AQ40 and Naxx both have much better armor bits for a holy priest but will never be suggested.
My snapshot is 4b2858fe13074559926c8fea141170f0 if you want to have a look