The problem with the Mastery shield is you ARE going to cast heals on the tanks fairly often, and, conversely, you're unlikely to need to cast heals on non-tank raid members more often than they're getting hit. That is, if there's a wave of raid damage, do you expect to hit one non-tank raid member with 2+ heals before the next one? Every now and then you might toss two on the same person, but will you A) do it before they get hit again? or B) do it twice within 6 seconds?
Realistically, if there's a single wave of raid damage every 30 seconds, and you're raid healing as a Healadin, you'll probably pop Radiance and then HL/HS/LoD people one at a time. In this case, you'll probably cast a single heal on each person, hitting LoD to "top off" groups as you go.
You MAY cast 2 heals on a single person, but only if they need the heals, in which case better to heal them, wasting the first bubble on them, and then move on rather than worry about the "wasted" bubble (if they're only taking damage every 30 sec, then only the bubbles you cast on the tail end of your triage circuit will matter anyway.)
So the only time you have to worry about overwriting a shield, realistically, is going to be on a tank, who's taking more constant damage. Even in that case, unless boss swings are REALLY slow, you aren't going to be out-casting them. You shouldn't be spamming FoL unless you HAVE to, in which case the wasted bubble is of secondary importance anyway. And HL/DL shouldn't be getting off faster than boss attacks too often anyway.
And, again, you if the tank NEEDS the heal NOW, you aren't going to go, "Oh wait, gotta wait...wait...THERE, now that the bubble's gone, I'll throw the next heal." That is NOT going to happen. Better to overwrite the bubble than not.
The exception is in cases where the tank is taking VERY small amounts of damage over a long time. But if that's the case, you'll be stopcasting or waiting between heals (for regen) anyway, in which case the bubble will get its full duration and you won't be clipping it with haste.
Basically, the Holy Paladin mastery, at present, isn't useful for tank healing except as a bit of a perk. You aren't going to base your healing around it. Likewise, you won't be basing your group healing strategy around the bubbles unless they increase the absorption amount considerably, increase the duration, or both.
The bubble will probably aid in group/raid healing somewhat, but it will be more of a passive benefit. "Oh, I don't have to heal him as much as I was expecting because the bubble absorbed some. You get a HL instead of FoL/DL, my good fellow!" Since you won't know the amount the bubble will cover (you probably won't be keeping up with the bubble amounts of 10/25 players in your head...nor could you case a heal on all those players within 6 seconds if you tried, regardless of your haste.)
So the bubble will be more of a perk, lowering how much you have to heal for, but won't likely factor into which heal you cast BEFORE they take damage. You'll just notice that after 30 sec, when the NEXT wave of raid damage hits, you have to heal 2-5 people for less. In this case, haste is BETTER because you'll be able to throw out more bubbles within the last 6 sec before the wave hits, meaning more bubbles will be active. Instead of 3-4 bubbles (non-haste HL/DL casts), you could have 4-6 bubbles due to haste!
...in this sense, haste scales POSITIVELY with mastery when you're raid healing, since you can have MORE bubbles up at the same time across different raid members.
Where haste scales negatively with mastery is when you're tank healing, and if you're tank healing, the bubbles will be next to trivial anyway unless they majorly buff the mastery/bubbles itself/themselves.
Now, as far as it goes, crit will NOT likely be overhealing in Cata. This is due to 100k health pools. :)
So I'd say stat priority will be thus:
Int (mana pool+SP+crit+regen...what's not to like?!)
Spr/Int until you feel comfortable with regen
Haste until GCD soft-capped
Int does SO many good things, it's far and away our best stat. Gem/enchant for it. Reforge Spirit. You may prioritize haste if you want until the GCD, but like hit for DPS, after the cap, haste isn't useful, so THEN you can pump your extra stats/gems/enchants into crit or MORE Spirit.
See, that's the real thing here, haste is better than a lot of things UNTIL the GCD cap. After that, you shouldn't stack any more.
...at that point, is it better to stack more crit for the extra healing/bubble size, or would it be preferable to simply stack more Spr for greater regen. More Spr means you can cast more FoLs if you need to instead. With crit, if you get the crit you need, yay! If not...corpse run!
Int > Spr till comfortable > Haste til GCD > MORE Spr or Crit (depending on play style) >> Mastery