I posted about Strength of Soul in light of the 4.0.6 changes to Power Word: Shield. Now we have the subsequent hotfixes to increase the cost of Power Word: Shield and to increase the value of Rapture. Is Strength of Soul still a useful talent?
I had concluded that against a single target, Strength of Soul provided a 10% increase in throughput and a 4% increase in efficiency. Assuming you still cast Power Word: Shield whenever weakened soul is done, Strength of Soul now provides the same 10% increase in throughput but actually ends up costing you some mana for that extra throughput, decreasing efficiency by 2%.
Of course Strength of Soul is also what lets us do heavy single target throughput when the fight calls for it. The Shield -> Flash Heal x3 rotation just doesn't work without it.
Strength of Soul is now a talent that lets you burn mana for throughput when you need to. Given the state of Discipline Priest healing, I think this makes it a worthwhile investment. But if you are not in a moment where high throughput is required, Power Word: Shield should be timed to maximize Rapture procs instead of being cast whenever it is available.