PW:S spam works exactly as you'd expect for Twins, but because of Weakened Soul and the way the damage occurs, Holy has stronger tools (empowered Renew, better ProM, CoH) for healing the damage.

PW:S spam is strongest against infrequent spike damage like Kologarn's Overhead Smash or Freya's Ground Tremor; it is comparatively less powerful against persistent periodic damage like the Twins aura ticks.

Hi priests
Sorry for my "frenglish" , could enlight me on somme different value of haste
if I want my GH go from 2.5 sec to 2 sec, unbuff.
I refer to Casting speed - WoWWiki - Your guide to the World of Warcraft

at level 80
% Spell Haste at level 80 = (Haste Rating / 32.79)
so 1% haste= 32.79
so 20 % haste = 655.8
so GH = 2.5 - ( 2.5*20/100)= 2 sec


an other formula gives
Haste Rating needed at level 80 =
((Base Cast Time / Desired Cast Time) - 1) * 32.79 * 100

for GH
((2.5 sec / 2 sec)-1)* 32.79 * 100 = (1.25-1)* 32.79 * 100 = 819.75

why this difference between these two value of haste?
for the same reduction of GH cast time

You need to understand the mechanism of haste :
Hasted spells have the following cast time (with haste between 0 and 1, as a percentage).

Hasted_Cast_Time = Base_Cast_Time / (1 + haste)

This means that your first computation is wrong : with 20% haste, you get a gh at 2.5 / 1.2 ~ 2.08. You need 25% haste, as your second formula tells you.

OK, Thanks;
I can calculate that a correct value of haste for a holy priest to decrease GCD from 1,5 to 1,3 is 505 haste.