To respond: on the live servers, no, Divine Aegis doesn't take overhealing into account. According to the patch notes, that's been changed in 3.1: DA will now take effect based on total healing, not effective healing. However, Matt's tests on the PTR have been inconclusive; he hasn't seen DA proc when he casts on himself at full health. No idea whether that's a cast-on-self issue or a not-proccing-on-overhealing issue.

That's a fair point – for some reason I'd misremembered DA as shielding for 50% of the healing done, not 30%. The lower number makes it much harder to reach the 10k cap, although I don't know what healing numbers for T8+ look like. I'm less reluctant to construct hypothetical scenarios, but since I don't have all the details it's hard to do so anyway. :)

I'm glad to see these two changes: it'll stack (makes sense with Penance in particular as I hated seeing 2 or 3 crits on that spell and having one or more bubble wasted) and it'll work off of total healing, not just effective healing.

I'm not sure I understand your concerns. While the absorption amount will be maxxed at 10K, I don't see how it'd be easy to reach that unless your target was taking no damage. I think DA would be consumed before it'd have a chance to stack that high. Of course, I have no data to back this up, and I don't want to construct hypothetical crit heals with hypothetical incoming damage to argue the point.

You're right that this is a direct buff to Penance. I agree 100% with you there. :) I'm also in agreement that if a shield stays up for very long, the encounter's not that hard to begin with. ;)

What worries me is that this is going to be used as fodder for the “Discipline priests are only single-target healers” argument. Although it's a counter-productive attitude to have, I know I'm not the only Discipline priest who's hesitated to heal a target with DA up because “what if I crit and I waste my DA?”. I know it's silly, but from my discussions with other healers it's far from uncommon, and this buff to DA removes that moment of consideration; even if my DA doesn't add to the amount absorbed, it does add to the duration of the shield.

Anyway, I guess the bottom line is that you've answered my question and given me some things to think about. Thank you!

I wouldn't worry one bit about the max amount of the shields. It requires 33K healing to be done, through crits, from that one priest, and if any damage is taken while he's building up his shield, the shields mitigate it. It's unlikely it will reach the upper boundary, and it is more likely the full effect of the shields will count.

DA duration is 12 seconds. It isn't rare for some tanks to avoid attacks for that long, but not a single blow during 12 seconds? not likely to be a tough encounter then ;-)
Still, should the duration be an issue… refreshing it by forcing some crits on the tank is hardly a bad idea. The crit also procs inspiration which can easily yield a 10% physical damage reduction. In other words: awesome synergy.