tested this out in BWL/MC http://db.vanillagaming.org/?talent#bxRGsVioobxtcs
worked great except I'm at 773 +healing in 8/8 t2 in my gear alone. The PI is for boosting lock/mage dps on encounters in raids.. the only crap part about it is getting familiar with timing the PI with specific casters and their CD's and ofcourse it being up for the boss. the issue I really disliked was PI being on cooldown between bosses if they are short between eachother. (used it on ebonroc, couldn't on flamegor)
the 3min CD really is a pain.
martyrdom comes in handy if you get mobs on you that do small damage (whelps from suppression room) as well as any 5 man content or soloing. the imp mana burn is very handy in pvp and AQ20/ZG for mana draining. it's not Amazing, but having mana burn on a faster cast time can come in handy when needed. I was thinking about dropping imp mana burn and/or martyrdom for 5/5 silent resolve IF it came down to those 2 talents being worthless for full threat reduction. (less fade use).
I wouldn't recommend ALL priests go PI spec until they have enough +healing from gear to compensate the healing loss from SG and SH from Holy tree. personally, 700+ healing is a good indicator to swap to PI.
we're gonna try 2 PI priests out to help ensure uptime on our selected caster for dps boost. anymore would be more work and less of a safety blanket for healing in my opinion.. (more to manage with your selected caster and such)
definitely test PI build out though and get with a caster or 2 that you wanna work with. it's easier to manage with a lock or a mage.. just make sure you use PI after they use Arcane power. if you PI before AP, it will overwrite PI.. but if you try to PI with AP up, it wont waste the PI (fail to cast due to stronger spell active).
very fun build though no doubt and worth the extra effort. your caster/s will love you too.
btw: force of will does not affect healing at all.. it is only damage and offensive crit. not healing.. so it is a dead talent for healing.
do not waste points into improve renew if you're thinking about going PI build. a priest with SG and SH would have more +healing on their renew than you with 3/3 imp renew in PI build.. it's not worth it.. the 5% crit will help make up some of the +healing loss from SG&SH if anything. if you want a designated renew priest with 8/8 t2, make them go SG and SH holy build and cover the highest hot coefficient for that spell. PI build is not for renew use. (sadly, I am only one with 8/8 t2 in guild cuz priest tier 2 never drops. rip) SG gives bonus to the gheal renew from 8 piece as well for more stacking.
holy nova and extra range for that and Poh will come in handy later on in AQ20 and other content. don't trade that range bonus for 10 extra heals per tick on renew! it's not worth the death!
mental strengths mana bonus applies after gear and consumables(distilled wisdom).. so yes, you will gain a lot more mana to help offset some of the loss from PI and lower healing per spell. you can check out your setup with the theorycrafting addon.. just click on the numbers next to the talents and it will give you the values with 0/5 SG and 5/5 SG. same with SH so you can see how much +healing exactly you are losing per spell.