Polymorph is not useless at all, it's a spell without a cooldown that also lasts for 3 times more than HW:Chastise.
A penance tick crits for 400, and the first tick is pretty much instant, meaning it's imo better than DP due to the fact that it's situational when or when not you will have the time to get the other ticks. Penance also has a 10 times shorter CD than DP.
Also I figure there must be quite a huge difference between the pugs in EU and in US
, since I can solo most 2-man D teams quite easily as disc, and I almost never can get anyone with me on O.
I don't know why, but it feels holy just hasn't got the same capabilities on O. Of course I've tried holy and I'm well aware of how to play it good, but the EFC usually dies in the first fear if the D doesn't trinket. If the supporting healers in enemy teams defense trinkets however, I just wait until fear is ready again.
And there really isn't a too huge amount of priests in the EU. I'm usually the only priest on H team meaning the shield is the most useful spell I can use.
You should have the time to Hfire-MB-Penance. Psychic Scream the target, Holyfire him (now it should break unless some absorb is up), then mindblast before he gets to a LoS-point and just penance before he moves there.
As holy, renew should tick for ~20 more, which in my opinion is completely worthless if you compare it to what only one single penance tick can do instead.
Flash heal heals for ~450 as disc, and a bit more than 500 as holy. A penance takes a little less than 3 sec to cast, meaning you get heals of 200-200-400 in that amount of time (assuming one tick will crit since your crit as a priest should be relatively high). 800 healing done in 3 seconds, and if say a flash heal as holy heals for 70 more than a disc one (crits for 140 more than a disc-crit), you'll get off about 6 flash heals in 12 seconds (if you also cast renew and PW:S in that period of time). If 33% of your flashheals crit, you get a 140+140+70+70+70+70=562 amount of healing done as holy. But then also your renew ticks for about 20 more, and 4 renew ticks heals for an amount of 20+20+20+40 more than disc = 100 more.
562+100=662, and
800>662.
I was nice now, and I didn't count the extra absorbtion from power word shield as disc.