So I've given the new set bonus a go in some raid fights since last time and here is my conclusion.
I don't like it.
I don't like it at all.
Like I mentioned before, Flash Heal is mostly used when being disc and not even then does it compare to the 2set t9 bonus. Eventhough Prayer of Mending is even better when being holy (thus making the 2set t9 -even- more superior to 2set t10 when being holy), it is still one of the disc healers main heals. I use it every cd and it always turns out to be about 20-30% of my total healing.
My flash heal could come up to this too when being disc (rarely when holy though) and yet Blessed Healing, the hot-proc from the tier, doesn't comprise of more than approximately 2% of my total healing. 20% of 25% Prayer of Mending is still twice the healing. Even more when I'm holy where Prayer of Mending easily can go over 30% of my total healing.
One could discuss variables such as overhealing (i.e if the extra heals from PoM were mainly overheal they would be unecessary although it looks good in the meters), but I think 20% extra from a PoM and 11% (33% from 33%) extra from a Flash Heal has about the same chance to be an overheal, so that isn't the issue.
This makes me conclude that 2set t10 is only better than 2set t9 for those fights where you honestly think you'll use Flash Heal way (and I mean waaaaay) more than Prayer of Mending. I can't currently think of such a fight in ICC, but Patchwerk in Naxxramas would've been such a fight. Maybe Gunship and Saurfang. Unfortunately (?) most fights in ICC are about alot of aoe dmg, and that is where Prayer of Mending really shines. Being able to greatly buff one of the priests awesomestest (estest) heals is just too good. Eventhough downgrading gloves makes me lose some 20+ spellpower, 10+ mp5 and other goodie stats, I still do it sometimes because I think that extra healing is worth it.
I haven't been able to try out the 4set yet, simply because I don't have it. Looking at it though it, just as 2set t10, looks to be better for disc than holy. It either gives you another Penance or Circle of Healing, and a disc generally uses Penance way more often than a holy would use CoH. And like I said before, it is more likely a disc priest casts that Flash Heal at all, that may reset the cooldown. The whole idea of being able to use Penance more often gives me a warm, fuzzy feeling though, so I hope it won't disappoint me like 2set has.
Yet again they want to fool me into using Flash Heal more than it is good for, but at least it is addable to the effect of 2set.
I still think it has some value to a holy priest though, because when things go tough and you need to pump out heavy aoe heals, this could probably come in handy. The difference being it will be useful for a disc priest in any situation, but for holy mostly for extreme situations. But since most raid encounters quickly turn out to be extreme, it can turn out to be useful to a holy too, like I said.