The extreme Priest prowess numberwise is just a result of IHC right now, and as many said the chances of that going live are probably ~nil. Most likely it'll only work on the actual HC-affected heal, a reasonable talent then, but not as overly strong as it is right now.
Game mechanic-wise, it's can't work like this since haste is on-cast while cost reduction (and HC consumption) is based on resolution. It would have to be something like "60% haste until Holy Concentration is consumed, or 20 secs".
Even if you could do this, it would effectively grant 16% * 60% = 9.6% haste over time. Which merely diminishes the overwhelming lead in single target healing, not eliminates it (see next reply).
Edit: Also, I don't know where to post this, but has anyone else noticed the huge disparity between priest heal values and shaman heal values?
I've been trying to point this out, but perhaps a comparison against where we are now might help (all values with 0/0/0 talent spec):
Greater Heal (Rank 7) has 13.3% more throughput and 1% more efficiency than Healing Wave (Rank 12).
Flash Heal (Rank 9) has 7.0% more throughput and 0% more efficiency than Lesser Healing Wave (Rank 7).
Greater Heal (Rank 9) has 31.4% more throughput and 63% more efficiency than Healing Wave (Rank 14).
Flash Heal (Rank 11) has 20.0% more throughput and 60% more efficiency than Lesser Healing Wave (Rank 9).
You're again failing to remember that spamming SL on a tank creates a healer who isn't healing. Damage to the raid did not drop, is simply spread out a bit, however raid healing drops by the value of 1 shaman, this is not an insignificant effect.
The problem isn't that raids 'run out' of healing. Rather, it's directing that healing properly. In a very real sense, 4 Resto Druids could heal any raid there is - if you could spread all the damage of that raid evenly amongst the 25 members of the raid. So if you've got an ability that mitigates damage on someone who is taking a lot of it at the expense of increasing the damage on those taking almost none, that's a major benefit - a big enough benefit to commit 1/25th of your raid force to doing nothing but transferring that damage.
Its power is on the order of a spammable Power Word: Shield, as of right now.
Spammable Power Word: Shield wouldn't really be that great from a PvE raid healing standpoint - it's less hps than spamming Flash Heal. The reason Power Word: Shield (and Prayer of Mending) aren't spammable is almost exclusively due to PvP considerations.
Firstly, the way SL is worded makes it sound useless. 'Nearby party members', so its not targetted?
This is the same wording you'll find on Chain Heal and Prayer of Mending, so I'd assume there is some algorithm directing it. Obviously not the same algorithm because you don't want the damage jumping to the lowest health members of the raid within range. My suspicion is that it will latch onto the highest health members instead. Whether it locks in at casting or chooses whenever damage is taken is anyone's guess.
In fact the 15% on water shield is probably worth more than 3 points in spell cost reduction for longevity, the earth shield bonus is just gravy.
Normally cost reducers are far more significant. 5% on Chain Heal (Rank 5) comes out to 54 mp5 if chain-cast. 15% on Water Shield is 7.5 mp5. Obviously these figures will change with WotLK, but it should give an indication of the relative power such talents normally have.
From what Ive seen raidhealing as a general concept is getting a major nerf (cooldown on CoH and Chainheal),
I hadn't heard that Chain Heal got a cooldown - I have yet to see any information on post-70 ranks of Chain Heal. However, Circle of Healing isn't really get "nerfed" so much as "changed". Instead of healing a group, it heals the entire raid. So a single 450 mana (pre-talent) Circle of Healing will be able to deliver 25,000 healing if your raid stands within 15 yards of one another. While this obviously changes the style of healing for Priests, it's not a 'nerf' really - and it only affects one of several options Priests have for multi-target healing (and all Priest multi-target healing gets +10% from talents in WotLK). Druids also get Flourish - another raid-wide heal.