Hello there fellow MG Priest!
First of all, Nillie has a great point. You are going to be fighting against some gear issues at this point, and that's to be expected. Without the meta, you need more spirit which is more stats you could have used elsewhere. Without the cloak, which is extremely useful for Holy, you're missing out on a lot of "free" healing.
Flash heal was your top heal his fight, and this is a bit interesting. Was it that you were healing the puddles this way? Don't get me wrong-- FDCL is a great talent for this fight, however it might be more efficient to weave a glyphed binding heal in (I found this to be extremely useful on this fight). Also, Halo seemed like it didn't do a lot of healing. I found that Halo was best used either right as the healing puddles came out (at max range it heals 2-4 around me to nearly full) or right after he goes down again when you get raidwide damage-- you can't do both because of halo's CD. If you find the raid needs a lot of healing both those times, Cascade is also a VERY solid choice.
Here you chose Halo. I found that DS was a better choice because the most healing intensive phase is when you're stacked inside the bubble. All priest talents currently have no diminishing returns, so considering this, during that stacked phase DS will hit everyone twice for it's full amount. Your CoH overheal is a bit concerning. Do not use CoH on CD. Look to see if the following conditions are met: Am I moving? Are there people in need of healing in different groups? If yes-- then CoH. If you can stand and cast, or it's not more than 3 seperate groups, sometimes a FH + GH combo, binding heal, or PoH is a better choice. PoH being the most mana/heal if it has the lowest overheal.
Here You also took Halo. We stack and swing around to the stack point whenever we get jostled around (by an add spawning, or by the sweeping beam). DS then does a huge portion of the heavy lifting as far as healing goes. Halo only did 5.2% of your healing, which is fairly pitiful.
Sha of Pride:
Another example of a poor lvl90 talent choice. Remember to change them out on a per fight basis, they should be doing at least 10% of your total healing done on their own. If not, they're likely not the right choice. For most of Sha of Pride, you will be stacked (unless people have swelling pride + more than 25 pride). Divine Star shines here.
Halo here too, and it only did 4.0% of your healing and 80.1% overhealing.
Halo was actually the best choice here, IMHO. And see, it did <10% of your healing.
Why are you 7 healing? Well, that's really a guild's choice but 7 healing is a bit excessive. This may happen on other fights too, didn't notice. But overhealing is a sure way to make a holy priest look bad. Everyone else gets to the healing first, generally. Halo only did 1.3% of your healing. We also stack and rotate around the area, DS is a good choice if you do this.
I don't ever seeing you use Void Shift? It's a powerful way to save a tanks life if needed.
Additionally, I never see you use Guardian Spirit. It's worth using if the tank is low on health, than not used at all. And I don't see you using Inner Fire OR Inner Will. I reccomend Inner Fire-- it's the best choice unless you're partched for mana 24/7 but really even then, Inner Fire scales with Spell Power. The more you have, the better it gets.
And no flask? I could be reading it wrong but... Flask is important. 1000 stat points. I consume Flask of the Warm Sun
You're truly fighting with a combination of factors:
1) Overhealing fights
2) Not enough gear/meta/cloak
3) Not picking the appropriate priest talent for the fight
4) No self buff usage
I switched to divine star! It's actually really awesome and more mana efficient from what I've seen.
As far as the gearing goes.. yeah, it could be a lot better. Should I reforge things differently or get different gems? I don't really think I need haste because in 25M I don't really use renew all that much for raid healing.
I wouldn't worry abotu haste, just aim to pick up a few SPI/Haste pieces as you gear up. Haste isn't as mana efficient, so you're probably in a good place right now since you have no meta. Your gear is going to be changing a LOT as you gear up. Haste isn't something to be avoided, but right now I wouldn't reforge into it by any means. Maybe if you really wanted to, the 3039 but the only really great breakpoint for Holy in 25m is the Lightspring one at 4716. Chasing extra ticks of HW: Sactuary is pointless at this point, it's just not worth it.
If mana doesn't feel like an issue to you, feel free to switch some gems to spi/mastery or spir/int.
I think you'll also find a huge improvement after getting the 2pc and 4pc tier. The bonuses are quite great.