You're almost certainly right that some +crit and some +haste is better than either extreme. I didn't intend to imply that Discipline priests should want gear that was all +haste all the time; my post was really a reaction to the Discipline priest guides I've seen that claim that the best strategy is to stack +crit as high as you can, with +haste as a spackle to fill in the itemization gaps. I think that attitude is contributing to a lot of misconceptions about Discipline priests, and I'm trying to correct it.

Like I said, I'm not actually finished with this idea yet, but I've been buried by work and Real Life and haven't had a chance to work out the kinks in it.

I'm aware that this isn't a realistic scenario or Discipline priests. The goal here was not to say that I think Discipline priests should just spam Flash Heal (I agree with you that that's ridiculous) but to provide a baseline for raw throughput.

I actually am not finished with the train of thought that I started in this series of posts, but I've been weighed down recently with work and Real Life, and I haven't had a chance to move on to the next station, so to speak. In short, though, my ideal Discipline priest strategy is to mitigate as much damage as possible with Power Word: Shield (in order to keep Borrowed Time up) and to supplant that mitigation with Divine Aegis (which now takes into account overhealing and will stack – which might cause some interesting confusion for mitigation meters – but which has a maximum mitigation). Penance is a big part of that plan as well. But like I said, I haven't finalized it yet, which is why I haven't posted more about it. :)

I hope that helps with your understanding of this post!

That would require some rework of my calculations *sigh*
But I bet it still shows that 'some' crit +haste is better than no crit + lot of haste or a lot of crit with 0 haste. IMO, the most interesting question is not whether this is true (the formula says the answer is yes) but if there will ever be need to gear for more crit than you get from intellect.

Of course, this is all assuming a FH spam scenario.
As for investment in intellect: yes, it is probably better than mp5. While this may go against the religion of many disc priests, even spirit beats mp5 when you have over 1200 intellect.

Do you mind if I ask where you got the item budget values for haste and crit? The table I was looking at valued them about the same.

This narrows the distance between the stats, though there are still far more facts to take into account. Eventually, a lot depends on the player's style.

I disagree with the maths as it doesn't take into account DA, for example:
– The crit does 150% of the healing
– DA takes 30% => 30%*150% = 45%
So you nearly double (195%) the value of a crit.

Secondly, you do not use the proper stat weights. For 1095 haste rating, you can get 1314 crit rating. Assuming you stick to the same iLVL of gear, you get more crit. So, we need to compare:
– 1095 haste rating, giving 33.39% haste, with a base crit of 17.95%
– 1314 crit rating, giving 28.62% crit, for a total of 46,57% crit

Using your formula you get:
– 150H, for your Haste set.
– 144H, for your Crit set.

That's 4% difference only. However, this isn't the complete picture. You're still not taking into account the uptime of inspiration. The uptime is calculated with the following formula:

Uptime = 1 – (1 – crit)^n
where crit is your critchance and n is your amount of spells cast within the duration of your uptime (15 seconds).

With 46.57% crit this is:
Uptime = 1 – (1- 46.57%)^10 = 99.81%

With 33.39% haste and 17.95%:
Uptime = 1 – (1 – 17.95%)^13.339 = 92.86%

This uptime provides a 25% increase in armor. Taking my guild's pally with 23587 armor, his armor increases to 29483.75 and his mitigation from 58.64% to 63.93%.

That's a 5% decrease in damage to be healed which would require a 5% increase in hps otherwise. The haste package only offers 4%, so you need 1% extra to get to the same value.

If you take into account that:
– Borrowed time gives a hard cap and a diminishing return of haste
– crit heals more for the same amount of mana, thus increases mana efficiency
– Divine Aegis procs increase the mana returned through Rapture

However, I would not canonize these maths just yet. Clearly, there are far more factors involved in healing than the example here comprises. At least, I hope I've indicated that Haste > Crit is not always true.

Haste gives the biggest direct healing throughput, but Crit shows more synergy with a bunch of talents. Even so, it would be wrong to think that one stat beats the other at all times. Just look at your hps formula:

<total hps> = <base hps> * (1+(50% + 45%) * crit%) * (1 + haste%)

Both crit and haste multiply the hps. This effective means that haste and crit work in tandem. Logically, this can be seen as:
the more haste, the more spells you cast, the more chances on a crit you can get.

While the exact balance of haste and crit may vary depending on the situation, at least this formula says that some crit and some haste will give better results than dropping either stat completely. Not using exact science here, but I bet that a comparable amount of haste as crit (say, both 20%) will score best. However, you start with your base crit from intellect, which for int stacking disc priests is quite high. That's why, at first, haste may give the biggest gain untill they have a comparable %.