I was always under the impression that improved regrowth gives +50% crit, so if you started with a 15% crit chance, you would end up with a full 65% crit chance after taking the talent. However, I'm having some trouble verifying this, and I don't currently have the game installed. Sadly, I believe regrowth is still very inefficient. Crit heals are often wasted due to overhealing, and there isn't much crit healer gear available.