Flash Heal

It's worth noting, that for Discipline priests this spell is replaced by Shadowmend at level 28. I don't see that information anywhere in the wowhead interface and spent some time confused about where my Flash Heal was supposed to be.

This is priests' quick, expensive heal.

In Cataclysm (as of 4.0.3a), all healers have the following types of heals:

Quick, expensive heal: 1.5 second cast, very high mana cost, high heal value.
Inexpensive heal: 3 second cast (usually talented to 2.5), very low mana cost, low heal value.
Slow, big heal: 3 second cast (usually talented to 2.5), high mana cost, very high heal value.

The mana costs differ very slightly between classes. Two of them have the quick heal cost more than the big heal.

Priest
Quick - Flash Heal
Cheap - [spell=2050]
Big - [spell=2060]

Shaman
Quick - Healing Surge
Cheap - [spell=331]
Big - [spell=77472]

Druid
Quick - Regrowth
Cheap - [spell=50464]
Big - [spell=5185]

Paladin
Quick - Flash of Light
Cheap - [spell=635]
Big - [spell=82326]

Again, not correct.

Flash heal vs Greater heal never comes down to raw numbers. it comes down to opportunity. You state that Discipline Priests should be using PW:S followed by greater Heal, without taking Penance, their 51 point talent, into consideration. Your views are based on the perfect scenario. This will never happen.

You keep mixing Discipline with Holy. Both are completely different in every way. Holy is a raid healer and will very rarely use Greater heal unless high, predictable incoming damage is present (patchwerk). You look at any guilds 25 man Parses from WWS and you will find that the main spells used are CoH, Flash Heal and PoM. Greater heal will be lowest.

Discipline is a Single target Healer. We should not raid heal as we do not have the skills or talents to do so. Main Tank healing is what we do. We Flash heal long enough to use Penance. Greater heal will not be used. even on patchwerk.

The debate of Flash Heal vs Greater Heal based on numbers
Greater Heal Wins

The debate of Flash Heal vs Greater Heal based on

1. 
   
2. Usefulness
3. 
   
4. Quickness of casting
5. 
   
6. HPS depending on deficit
7. 
   
8. Amount of procs turning (Flash heal into Instant casts).
9. 
   

Flash Heal Wins.