Oct 12th 2010 5:01PM In terms of efficiency, GHW at high gear levels will only begin to match HW at low gear levels. By efficiency, I mean heals per mana (hpm).
HW at 5k spellpower works out to be able 2.75 hpm. At 10k spellpower, it would be about 3.5 hpm.
GHW at 5k spellpower is 2.154 hpm. At 10k spellpower, it's 2.769 hpm.
However, there's a bit of risk in casting a high-mana cost spell that might be overheals. If someone else beats you to the punch, you could cast a 6.6k GHW for no benefit -- all overheals. If it had been a HW, you would've only lost the 2k mana cost.
As it stands, you will need to think about what is appropriate at what time. If you don't want to run out of mana, you're going to want to avoid overheals and use GHW as a situational spell (at least for some time).