T9-4 pieces Bonus for discipline.
If Divine Aegis goes from 30% shield on crit to 40%... (crits go up to 210% raw).
It's an increase in throughput of 4.3% at 40% crit, and 5.1% at 50% crit. Assuming: no overheal, no PW:S.
The more you overheal, the better it becomes though, because DA on a tank doesn't overheal (and the 10k cap is pretty far if you're the only disc priest assigned).
At 40% overheal, no PW:S, it's a 6.7% increase (at 40% crit) or 7.7% (at 50% crit). (Don't start to believe overhealing is good though...).
Now if PW:S absorption effect accounts for x% of your total healing. The increase from T9-4 bonus is multiplicated by (1-x).
For example, if your PW:S did 30% absorption (average when you cast some shields on the raid), at 40% crit, 40% overheal you get a 6.7*(1-0.3) = 4.7% increase in throughput.
Notes:
I am not bothering with renew, or DA shields which are wasted.
I took 40% overhealing, because it's the maximum overhealing value I had as disc. It should be lower.
If you don't like my examples and arbitrary numbers, make your owns.
40% crit, 40% overheal, 10% PW:S absorption.
0.6*(0.6 + 0.4*1.5) + 0.4*1.5*0.3 = 0.90
0.6*(0.6 + 0.4*1.5) + 0.4*1.5*0.4 = 0.96
((0.96 - 0.90)/0.90)*(1 - 0.1) =
6.0%
40% crit, 20% overheal, 30% PW:S absorption.
0.8*(0.6 + 0.4*1.5) + 0.4*1.5*0.3 = 1.14
0.8*(0.6 + 0.4*1.5) + 0.4*1.5*0.4 = 1.2
((1.20 - 1.14)/1.14)*(1 - 0.3) =
3.7%
50% crit, 40% overheal, 10% PW:S absorption.
0.6*(0.5 + 0.5*1.5) + 0.5*1.5*0.3 = 0.98
0.6*(0.5 + 0.5*1.5) + 0.5*1.5*0.4 = 1.05
((1.05 - 0.98)/0.98)*(1 - 0.1) =
6.9%
50% crit, 20% overheal, 30% PW:S absorption.
0.8*(0.5 + 0.5*1.5) + 0.5*1.5*0.3 = 1.23
0.8*(0.5 + 0.5*1.5) + 0.5*1.5*0.4 = 1.30
((1.30 - 1.23)/1.23)*(1 - 0.3) =
4.0%
Edit: Actualy, 50% crit even in T9, might be overkill...