Serenity heals for 40-50% of a tank's health bar. The only spell that heals more than it does is Lay on Hands, which is a self only 10 minute cooldown.

We have ways to increase throughput when needed. Holy can talent to have your holy words give a 15% healing bonus. Holy's weapon has a 25% healing buff that can applied to the tank. Holy has a 40% healing bonus cooldown. And Apotheosis last 30 seconds (which is an eternity on 5 man boss fights) and massively increases throughput by letting us spam holy words.

On top of that, Holy Priest has the best Mastery of any healer, as Echo of Light is just a flat out increase in both efficiency and throughput, and works at all times and with almost every spell. Even better, it synergies with all of those +healing% increases. You get the benefit of the +healing on the initial cast, but then it causes the Echo of Light to be larger, so it's a bonus to the bonus.

End result: Holy can really dish out large number of healing when it needs to.