There are few things u need to know that haven't been said yet:
1. Get a rapture timer. This is such a huge help in keeping up with your mana. If you are just trying to guess when rapture is gonna be off cd you are always gonna miss out on 3-6 seconds. I'm using Ingela's Rapture Timer for this purpose.(I see you already got this, but I leave it here as a reference to other discs looking for a easier-to-configure addon)
2. I don't bind IF to anything, but use it manually. This is because there are moments where you don't want to waste the enhanced critchance on a normal gheal. Say your tank is about 80% and you know there is gonna be a bigger hit incoming, but you are getting off a gheal before. (But this is not really important, you can go with that as you please.)
3. Already when I was raiding t11 and t11hc(and your gear is kinda up there) I tried to get rid of every piece that had spirit, simply because I didn't need it. If you are able to time your shadowfiend with BL/TW/Power Torrent/Tailoring procc etc. You are gonna have enough mana.
Despite that Tsunami is great and I'm still using it progressing in FL hc ^^
The t12 2pc bonus is really good and regs for 412 mana every 5 seconds on my goblin. It's important to note that it's healing a percentage of your base mana and not your total. From the moment you got that I'd advise you to forge your spirit into another stat that suits your playstyle.
4. I believe manaregg is calculated by sqr(int)*spirit*const (correct me if I'm wrong), but as u see int also enhances your manareg. So by that int helps your mana with: rapture, shadowfiend, hymn of hope, innervate(if u can get it), and maybe something I'm forgetting right now.
5. GET RID OF THAT MALORIAK TRINKET. It sucks hugely for disc. Anything w/o int is a huge loss in every way you can think of
6. haste <-> mastery depends on a lot of circumstances. I've got my top WoL placements for shannox hc, in average-below average gear at that point, with stacking mainly mastery, or similar results for Raggi nhc. But on other fights(ryo hc, majordomo hc, etc.), which are more grp-heal intensive you are having troubles keeping up. As others said try it and find out what suits you. You may not be able to top the group as fast as other healers with mastery. But with the help of your huge DAs, you dont have to fear them dying
7. Can't comment too much on AA/Atonement heal, but I've always thought haste is valued more in these specs just because you can't improve heal by mastery, but please correct me if I'm wrong
8. Always remember the haste buff a shield is giving you. Be it not casting flash heal or PoM after shielding, or casting PWS right before hymning. Btw does someone know how Borrowed time works with the dot from holy fire?
9. Get a better enchantment for ur boots