On the subject of hard modes, most of them seem to be of the form "stack extra DPS and cut healers", so that's a situation where you might need more mana regen. However, they also have the form "this deals more damage", so you in theory need more throughput. Mana efficiency and throughput are always at odds, and I believe hard modes need to be analyzed on a case-by-case basis. We can't just say that "all hard modes require a greater focus on gear aspect X".
You're in love with spellpower. I'm not. You've made a lot of assumptions about things in my post based entirely on the idea that spellpower is more valuable than regen.
I'd like to take a step back here. Spell power is just a means to an end, just like spirit, crit, haste, and m/5 are. There is always a tension between how much throughput you have and how much longevity (regen) you have. Shorter fights tend to favor throughput while longer fights favor longevity. From that framework, I suggest Spell Power is the most efficient stat because it increases both Throughput and Longevity. If you could spend 2 itemization points on any combination of spell power, int (mostly all regen), and haste (all throughput), 2 points in spell power (roughly 2.1 spell power) will beat 1 haste and 1 int for both throughput and longevity. That's not to say there aren't fights where 2 int will be better (vezax) or 2 haste will be better (razorscale), but 1 haste + 1 int is never the right choice because 2 spell power beats it on both throughput and efficiency.
So when I hear you arguing for both haste because it increases Throughput and Int/Spirit because they increase Longevity, I'm dubious. You can't argue both sides. Either Throughput matters more, meaning you focus on spell power / crit / haste, or Longevity matters more, meaning you focus on spell power / crit / int+spirit. I wonder if the reason you are running low on mana is that you haven't focused on spell power. It's easy to fall into a trap where you get a lot of haste, which makes you feel low on mana, and then you try to compensate by stacking mana regen. Unfortunately this produces a situation where the heals are anemic, and you need to work a lot harder to keep everyone alive.
Related to this, it's worth noting that we as priest might need to start collecting two healing sets, one focused on mana efficiency and another on throughput. I think part of this dispute comes from applying a "one size fits all" gearing methodology to the variety of boss fights in the game. Neither a "hit these arbitrary thresholds" nor a "follow these arbitrary weightings" heuristic can do justice.
As an aside, your argument on the flasks was specious. You don't compare 65 intellect directly to 125 spellpower. Even if you did, it comes off looking pretty good. Flask of Distilled Wisdom is worth approximately 57 Mp5, which definitely owns the Mojo flask, which is itemized exactly the same as the Frost Wyrm. So if Frost Wyrm = Mojo in itemization points, and Distilled Wisdom is more regen than Mojo, then it's a better option. I'd rather pick up 65 intellect from a flask and use spellpower food to help make up the difference than use spellpower food and be unable to pick up regen from another source.
But we don't make decisions by itemization points, because we know that things with similar item levels can have vastly different utility. Even if you wanted to argue from itemization points, 65 int < 38 m/5, so if we followed that logic, we would falsely conclude Distilled Wisdom is worse than Pure Mojo.
At any rate, this is another aspect of throughput versus longevity. But this one clearly favors Frost Wyrm, because taking Distilled wisdom makes you lose 2 spell power for every 1 intellect you gain. If you want more longevity, it's better to make that sacrifice in gemming, where you only lose 1.15 spell power for every 1 intellect you gain.
Your comment about "Spell Power > Crit > Haste = Spirit = Int >> M/5", if applied directly to gemming, is basically false. Gemming is the perfect situation to take your overall gear level, and balance out the stats.
As far as the soft haste cap, I'm sticking with it. Having ~ 21% haste raid-buffed is a nice number. Obviously more will help, but depending on your use of FH and Serendipity
But why is gemming the perfect situation to balance stats? Every other class in the game uses gemming as the opportunity to stack whichever stats give them the best returns. I know that there is a theoretical mathematical optimum where all stats are roughly equal in value. And yes, clearly the more spell power you have, the better returns you get from crit and haste. But I haven't seen any math proving that the current gear level just happens to be in this optimum. Rather, the math I've seen says quite the opposite-- some stats really do provide better returns than others, and they should be stacked.
Similarly, I confess I'm uneasy about putting the "21% haste soft cap" in the root post as a "really important guideline" without any math to back it up. I know you are sticking with it, but I prefer my choices to have a mathematical backing. If there's math to back it up, great-- we should post that. If not, then it seems like a harmful guideline that just encourages inefficient gearing (from a mana standpoint).