Just to make things clear : haste build is not better than the crit build. Imo, If you are feeling like you can manage your mana pretty well and never find yourself going oom, you should start getting more int in place of spirit instead of reforging haste. Also don't forget that your niche is absorbs and with haste instead of crit, you have less absorbs (excluding the spirit shell haste breakpoints here) so you are less useful for the raid.
Pom shouldn't really do your most healing as disc. You might want to try prioritizing some other spells. You should have more atonement healing on that fight considering penance can be cast while moving and holy fire is instant. Overall seems to me like you are not using skills at the right time. (maybe you are trying to heal with poh or casting a lot of direct healing spells? On this fight don't poh unless you are stacking shields and as a disc %95 of the time you should be dpsing the boss to heal people instead of casting single target heals on them.)
DI is not the best choice for this fight imo, 1st quake you have no adds & plenty of time so SS should be capped, 2nd quake you can combine PI & PW:S to blanket the raid with shields without having your mana pool destroyed, after that use SS as much as possible, although movement can make it a bit hard to effectively stack it.
I definitely wouldn't be using DI this fight.
I went with Twist of Fate and had a 55% uptime on it. Help kill the bats/turtles and hit Tortos and atonement will be doing a lot of healing.
we always 2 man heal this fight with Disc priest (me) and/or Holy paladin / Resto shaman. the best thing u can do it use SS on first stomp Second stomp u use PW:S third u use SS again and so on. use Halo for that fight for Burst healing after each stomp (works alot better than star and cascade). and im on 85-95k Hps Without whoring if im pushing all i can do around 100-105k HPS as disc on that fight
And i use Haste (3039) > Crit > Mastery > Crit