if you have the gear you should aim for the 13k haste breakpoint, maintain a min of 16-17k spi and rest mastery, only in pretty much BiS gear should you go for the 18k breakpoint but the 13k breakpoint should be more than reasonable in 560 ilvl
for 25, you could probably take just about anything and still do fine, but haste has the strongest throughput and least overhealing at the cost of needing more spirit to support it compared to crit and mastery which at this point are both about even and both have their own overhealing issues and which makes you better on meters depends on healer comp and the fight (however, always remember that your 90 talent doesn't scale with haste so you don't really need too absurd of haste)
for 10s, really high spirit and a high renew breakpoint, then crit just because renew is going to be quite a bit of your healing, making mastery significantly weaker
it sounds like you mean 25s though, so try to keep at least enough haste that prayer of healing is at least around 2s imo (20% buffed, I think that's about 6k), but 90 talents are bigger parts of your healing in 25s, and pre-casting is more important, so you can drop more haste if you feel like it, but I'd at least try finding out whether or not you're able to pull better with different amounts of crit and mastery, but on this patch, it's really situational in practical situations, and I haven't talked too much to my friend about gearing for priests in heroic 25s lately
Pre-casting is a very important Holy skill. When to start casting before big damage/have FDCL procs up, DI up... And again when NOT to be casting.
I prefer to take Int and Int/mastery over more spirit/haste as Holy. I pre-cast PoH when I know I'll need it, and I'm very watchful of wasted PoM. I generally run 12-13k spirit as Holy and do well, but I'm 25m and rarely use Renew. For my LMG procs I'm using PoM, Lvl90 Talents, and sometimes glyphed binding heal if I have hero/lust up.
You're going to at least hit the 2nd lightwell BP in the high 4ks (run over to Howtopriest for the table, or totemspot... iirc it's like 4721). After that, go wild with Int/mastery, mastery, and Int.
Int is going to hit harder upfront, but mastery is better if you can't always be actively casting and aren't overhealign the nsot out of things. If you have 6 healers, int imho, will do you better. 6 healers and EoL do not play well together unless you enjoy being the overhealing master.
yah, also remember for high burst fights that you can reforge to like a large amount of haste for going rambo on/off high throughput healing when needed, it costs more mana and is less viable than it was from BC to Cata due to priest mop 90 talents and was somewhat nerfed in cata due to the loss of the 5 second rule, but now, around the lightwell breakpoint is nice just because it allows better movement and more casts for stuff while not hampering mana issues too much (like seriously better movement, really low amounts of haste, you start to feel like you single-handedly wiped the raid because you canceled your massive prayer cast to move out of shit)
I don't do 25s, but I'm going to assume running out of raid cooldowns on the mogu minibosses is hell, good luck with that