In 25 man heroic progression from my experience, any healing spec can excel at any fight. Some specs are predisposed to the success, but a competent player who understand how healing works now can easily help their raid survive and succeed in many ways.
Don't forget, healing (HPS) is not Damage (DPS.)
Your "numbers" may not always be the most indicative of success, as there is a limited amount of healing that needs to be done.
If you find yourself under preforming and are confident it isn't your skill, then it could be your raid is outgearing the fight, or your healing comp would due with a few changes.
Just because absorbs get their ticket punched first doesn't mean Holy is less viable.