Again, please be much more conservative with the quote-splitting.
Your making the usual theorycrafter mistake when thinking about tank healing, which is ignoring overheal. Even if you cast nothing but 1 PWS every 15seconds, which is not the case due to the divine insight.
Since PI has no mana costs... why on earth would I spec DI?
Every heal except PWS is highly likely to overheal. SpS much more so due to its 8s duration. Heal you will mostly be casting when the tank is full after SpS and the majority of them will overheal partially or completely. For the same reason a lot of the FDCL procs overheal.
Why would FDCL procs overheal? They stack a 20s buff which refreshes. It's your choice to use it.
Looking at disc tank healing on live you are looking at least 20-30% OH and well over half your casts overheal. If there is two healers on the tank even more so. Thus the spellpower bonus from IF is almost completely wasted.
If SP is wasted, mana conserve is wasted as well...
Tank healing with inner will means that casting 1 PWS every 15 seconds is equivalent to 480mp5, with completely negligible loss in HPS. Actually IF right now makes things a lot worse, because it makes it slightly harder to absorb a PWS. Try it out and you will see, you will end up with a ton more mana at the end of the fight.
Willing to try but how on earth would I end up with "a ton more mana" given that I sacrifice 5% Healing for 15%less mana, thus only gaining a raw 10% on instants, of which I only use one - and that just to get mana...
In the finished product DI is likely to be functional so we will probably be able to cast PWS more frequently
We would be - if we'd spam SpS a lot. But then again, at the current state: Why on earth would we want to cast PW:S often? And forfeit PI mana savings just to do so? (PI not only adds SF ticks, it costs nothing AND reduces mana costs by 20% - you'd seriously give that up for the off chance to get an additional shot on the most expensive heal in our arsenal?
For AoE healing on live I am permanently in inner will since 1:1 PWS/PoH is more HPS than straight PoH even with archangel on.
Why would I ever skill Archangel at the moment?
Add to that the fact that a lot of PoHs are cast on groups which are either competely or partially full and PWS/PoH HPS is dramatically higher than straight PoH spam. Since inner will allows me to maintain a higher PWS to PoH ration I am in IW permanently. If I cast 2:1 PoH/PWS in MoP, I will also be in inner will for aoe healing.
Instead of arguing here I urge you to try my strat out in a dungeon (as disc ofc) and post your results.
Last time I tried I went out of every boss encouter with 80%+ Mana, still applying for Holy spec. (And I got 372 gear+ what I picked up during that runs) so that kind of test unfortunately won't help us at all