There are some people around here saying that you shouldn't break t14 4pc until you get t15 4pc... take that with a grain of salt, I personally can't comment on whether or not that's accurate. But, I'm sure someone will be along shortly (eventually ;x) to drop the math on dat @ss.
Have you checked EJ? I'd imagine that you could find something there to eliminate all guesswork.
Unfortunately, there is not currently a dedicated Holy expert. Ret and Tank, yeah. But you won't find any greater amount of data on EJ for a pally healer than you can find in this forum.
Anyways, T-14 is incredibly powerful, and T-15 is not. With the increased drop rate on older LFR's (be sure to get some Elder Tokens to help out by snagging chests in the Treasure Scenario), you should have a decent chance at finding gloves and leggings for T-14 (keep the chest as offpiece, since it's your highest iLevel item).
T-14 > T-15 522ilevel four piece > No set bonus.
T-14 is about 8-10% throughput, varied per Holy Shock usage. You also gain a lot of mana efficiency and mobility. T-15 is only desirable on fights where you need stacked raid healing and your throughput can't manage. All else, it's only about 2-3% throughput increase. Choosing T-15 over T-14 is like choosing SS over EF, or HA over DP. It's a complete loss of HPS and is only helpful to counter one specific type of incoming damage... and you'd still be better off just reforging out of Spirit and into other stats if that's the case.
One caveat: Since healadins are still using T-14, Blizzard may end up nerfing it (now that it's not competing with PvP 4-set) or else buff T-15, so don't pass up on opportunities to snag some tier tokens.