I have to admit, my shaman rarely used Greater Healing Wave during 5-man heroics. I have it in a macro with Nature's Swiftness, so I "saved" it for when I need an instant big heal. This has been a mistake. With the Rapid Currents and Tidal Wave buffs from a Riptide, the first GHW has a 1.1 second cast time, the second GHW hits in 1.3 seconds. When incoming damage is high, I can Riptide, GHW, GHW, Healing Surge and then repeat since Riptide will be ready again. This rotation puts out a ton of heals very fast. During normal damage, I can Riptide, HW, HW, HW, repeat. The last healing wave will be slow, but the sequence is very efficient.
I have haste in every socket. I have reforged when possible to add haste. I don't have top end gear, but I imagine it's possible to get those GHW's down to 1.0s.
I recently skilled up my Herbalism to 450 to check out Lifeblood. A macro activates it when I use GHW. When it's available, I can Riptide, GHW(1.0s), GHW(1.2s), GHW(1.7s) matching my Riptide cool down exactly. The haste buff lasts enough time for about 3 sequences.
The shaman is a true Big Healer.