Several responders posted that this is partly the fault of Blizzard as they don't take into account shielding and mitigation to healing scores in the combat log. Daerilla of Spirestone intimated that she did not want to see mitigation included in healing figures, because then people would take notice of how overpowered Disc Priests are in raids. There were some less-than-kind comments about the Raid Leader's competence.
Raid leaders, be sure to give credit where credit is due. Trust that your healing leader and raiders know what they're doing. The raid leader should have a decent understanding of class/spec roles, and adjust their expectations respectively. If the GM is in charge of making healing assignments, be sure to give healers appropriate assignments. Not based on HPS, but based on healing abilities. It some cases it might be best to give a healer the opportunity to dole out orders to the healers.
As Mike Schramm pointed out, basing "performance" off healing meters can lead to doom for a raid. This practice supports over-healing and potential mana issues. When healing becomes a competition healers can lose sight of their assignments and slow down or even wipe the raid. As always remember to keep meters in perspective. It's not about being a star, or a prima dona, raiding is about working together as a team to accomplish a common goal.