depends on how you time the enrage windows. shadow priests have at most 2 DP's and vampiric touch, and a holy priest can potentially have power infusion up for each enrage window.
there's a log someone posted on the blizzard forums
http://www.worldoflogs.com/reports/r...655&target=178 where you can see that their hpriest smashed their spriest on boss damage, as well. their hpriest was going for twist of fate instead of power infusion, but PI would be more sensitive to changes in the spacing between enrage window. idk what type of strat they used for the boss, but i usually do 95-100m on the boss (as shadow) when we do this fight.