On Monday I looked at one way to liquidate excess mana: by substituting an intense casting rotation for a more regular one. Another way to liquidate mana is to simple cast more spells. If we simply took our excess mana and converted it into Power Word: Shields then the rankings for secondary stats would be Spirit (41.5%) > Mastery (28%) > Crit (12%).
If we instead used the extra mana to cast half shields and half renews - thinking that we are probably squeezing these extra spells in while on the move - then the values wouldn't be much different with Spirit (41.4%) > Mastery (23.5%) > Crit (14.7%).
But I feel that this is probably less representative of what is really going to happen in a battle, since encounters are designed around the idea that you will cast spells pretty much as often as you can, but in reality we don't have 100% active time on our log reports, so we can definitely squeeze some more casts in there. I was using a figure of 130k excess mana to spend, which would be enough for 28 shields, or about 38 seconds of extra casting over a 5 minute fight. This is surely an unattainable figure. 38 seconds would mean increasing your active time in a fight by 12.6%. Since my active time is usually around 87% this is getting a little too close to 100% to be believable. In addition to this, the reason we use intense healing rotations is because there are periods that warrant intense healing - even if these periods are just the result of a long time with dodges or critical heals. We can't make up for that loss by simply casting more shields during the lighter damage times.
Another option is to take a half-and-half approach, where we spend half of our excess mana on intense single target healing and half on extra shields and renews.
This result looks a lot like the result from Monday but with the numbers even closer together: Mastery (31%) > Spirit (28%) > Crit (23%).
Of course this doesn't mean that it's time to reforge spirit into mastery. In the case where we are simply running out of mana by the end of the fight, more mana would allow us to continue our normal 6.92 mana efficiency rotation. In this case Spirit (40.6%) > Mastery (18.7%) > Crit (16.5%). So it's a real blowout in favour of Spirit. If such a situation where actually winnable, it would mean damage output was low enough that the full rotation wouldn't be necessary, and mana efficiency would go even higher, making the benefit of spirit relative to the other secondary stats even more extreme.
Spirit has a kind of diminishing returns. Once you have enough mana to cast a baseline mana efficient rotation over all available times during an encounter, the value of spirit drops off. Similarly because an important part of what Intellect does is provide you with mana, the value of Mastery and Crit relative to intellect increase when you pass that point.
But of course determining where that drop off point happens is pretty difficult. Fights are not simple enough to be easily modeled all the way through. Also, I haven't talked about haste at all. Haste increases throughput at the cost of mana. But if I am trying to compare spirit to mastery through estimations of what extra mana can do for us, then surely I can try to fit haste in there somewhere.
When I have some time in the next week I'm going to build myself a spreadsheet dragon and discipline priest and have them face off in a tug-of-war over tank health. That will provide a better picture of how haste fits into the overall scheme of things.