This post is basically part two of yesterday's post so reading that first is probably a good idea if you haven't.
I finished off talking about the problems with a mastery rating that only affects one spell by saying that I hope the developers realize that the one spell mastery is the real problem and change that, rather than trying to change the power level of either the mastery or of Power Word: Shield when the problem rears its head.
Using Spell A and Spell B I just want to talk about why you can't solve the problem by tinkering with the numbers, but it has to be solved by redoing the mastery. In my example, Spell A is a two second cast for 20 damage and Spell B is a three second cast for 25. 25% of the time Spell A procs an instant cast of Spell B. I pointed out that if Mastery only affects Spell B and it is balanced with haste then it will get out of hand and eventually you'll just spam Spell B and mastery will be overpowered.
So lets compare how the numbers would actually work out. I was using the example of having 10 mastery which would give a 58.8% bonus to Spell B. First I want to show why this is "balanced" with haste. One mastery takes 1.4 times as much rating as 1% haste, so we should compare 10 mastery with 14% haste. If you do the rotation you are "supposed" to and cast Spell A until you get a proc, then use your proc, your basic rotation does 105 damage in 9.5 seconds, for around 11.1 dps. Put 14% haste you instead get 105 damage in 8.33 seconds, for 12.6 dps. If you get 10 mastery, increasing the damage of Spell B to around 40 then you get 120 damage in 9.5 seconds, again 12.6 dps.
The problem, as we noted, is that the mastery threw out the balance between the two spells and created a different rotation. The alternate rotation is worse with low levels of mastery and better with high levels. Once you cross over the threshold where it gets better, mastery becomes a dominant stat. At 10 mastery the spam Spell B rotation gives 13.3 dps, so it's 5.6% better than the rotation you are supposed to do.
What if we nerf mastery so that the rotation of spamming Spell B doesn't get out of hand? Easy enough to do. If reduce mastery by a little bit then we could find an equilibrium so that 14% haste and 10 mastery give the same dps. The problem is that this equilibrium would exist for those exact numbers only. If we wanted to reduce mastery so that 10 mastery gave 12.6 dps at most, then we'd need mastery to give 5.12% to Spell B per point.
But then what about a few tiers later when we have 15 mastery or 21% haste? The mastery option lets us spam Spell B for 14.7 dps while the haste option does only 13.4 dps. We nerfed the mastery massively but it ends up almost 10% better later on.
We could nerf mastery to make sure that spamming Spell B never became a real option, but that would mean making sure it never got above 36.9 damage per cast. If we know the most mastery people will be able to get in the expansion is 15 then we can set the value of mastery at 3.17%. So then how does 10 mastery compare to 14% haste? 10 mastery would give only 11.9 dps. It would result in overall dps being 5.5% lower than the haste option. Haste rating would be about 87% better than mastery for increasing your dps.
If instead of reducing mastery we reduce the spell, we'll either just recreate the problem in a more extreme fashion or we'll create a situation where the best way to play is to stack haste and just spam Spell A, ignoring the procs.
The numbers just don't work out. Even at a specific gear level, we can reforge, regem, re-enchant and get new items. Discipline priest mastery only really makes sense at extremely high crit levels. If we were critting 60% of the time then it could be affecting our non-shield spells more consistently. There would still be a significant imbalance in how it affected the spells, but it would be much less pronounced. On the other hand, if we could crit that much, why would we not just reforge our mastery into haste and stop casting Shield altogether in favour of spells that can actually benefit from crits?
Mastery for discipline priests, as it is, is unbalanced. In future tiers - or possibly right now in 25-player raids - it will go from being unbalanced to degenerate, and the only way they will be able to stop its degeneracy, without a redesign, is by making it useless.