*cough*
AOE Aura ticks, which are the only consistent AOE damage in fights tick either every 2, or every 3 seconds. This means that unless someone is hit by something other than AOE aura ticks PoM will never fall off within the cooldown. It's actually exceptionally rare even when bouncing it off a tank. For 2 second tick auras PoM will use it's last charge at 8-10 seconds. For 3 second tick auras PoM will use it's last charge at 12-15 seconds. Cooldown = 7 seconds. This means that under normal circumstances using PoM 'on cooldown' results in losing 20% of the maximum healing, but in some circumstances it can cost you up to 60% of the cast.
The standard safe timing is around 10 seconds. Most Auras are 2 seconds, which lines up the 10 second mark as fairly failsafe. That said if you are paying more attention you can cast it more often than this as your PoM may be eaten by another Priest's, or your targets may take extra damage which will cause it to fall earlier. Casting PoM on cooldown every time is bad practice and is costing your raid healing.
The reason haste is preferred is that it allows you to spread your healing out, which in general offers the same net healing increase as SP while also reducing the likelihood that you will be over-healing. The main concern here is how your cooldowns line up and how many Renews you can have active. You also don't need to worry if you're only dealing with 60-100 Haste, so I'm not sure why you'd complain. Just switch a single item, not an entire set. The difference in haste caps between Holy and Discipline is 257, not 60-100. If you're dealing with that it becomes harder.
I think your problem is that you're saying you don't cast for the entirety of fights. If you're not even being an active healer, then why would you bother min/maxing? Work on the laziness, then work on the gear. ICC25H can be healed in 232 gear if you have the ridiculous buff up (see: everyone) and pinky is hardly challenging to begin with.
Haste is preferred for three reasons though:
1. Increases flexibility.
2. Allows more targets to be healed simultaneously.
3. Tends to reduce overhealing.
ps: Still retired. :P