Ok first off, I'm epixheals of shattered hand...for the horde of course.
Announcing that I'd first like to say that I've been playing a priest for 5 years now, I'm no "pro" but I do top hps charts of every encounter that I'm in 5-25 man dungeons and raids. I average around 18-25k hps. I am a holy priest, I've been seeing posts about haste over mastery, and I must say it's what fits your playstyle, if you are an ape raid heals like I am, I disagree with haste. The reason being poh, coh and your aoe holy word heal and hot 6 ( if your glyphed for coh) if not the entire raid (holyword/poh) only one of these that is not instant cast is poh, that being said. My first prioritys are exactly as follows, very rarely with Any exceptions. Drop lightwell 30-60 seconds before fight, click chakra before fight, let it chill until encounter starts, throw pom on tank to activate your holy word drop holy word where the Majority of players are, most time in ds they are stacked in a tight enough radius to hit everyone but a stray. (if you have a 4 peice set for ds, it extends the duration of you holywords. Making the down time waiting for cd to reset on holyword, like 5 seconds, damn near spam able.) afterwards I watch and coh, or poh, throw renews like crazy ( even renew adds additional hot if your glyphed and talented it is very wise) always have hots on 8/10 players if not 10/10 and mastery only increases the hots strength. And if for whatever reason people need extra heals, it is ok to throw a flash, heal, or greater to them, just keep your aoes going at same time and or afterwards. As for pots I use conc myself, and as for this debate over veiled shadows, it is worth it, I use my shadow fiend 2 times every fight, use it at start of fight only a minute in, so yes you can go "wild" with heals for first part, shadow fiend then arcane torrent every time it's up, I get anywhere from 3-4 of these in per fight. As they said, before you use your hymn or conc pot, CALL it out!!! Thanks guys hope thus has helped check me out on shatt hand horde side, epixheals is the name.