As much as I'd hate to say it, I did some testing, and I found that the mana returns for Archangel and Dark Archangel are reversed.

Archangel (Evangelism) is healing for 25% of my max mana, while Dark Archangel (Dark Evangelism) is only returning 15%.

This'll be a big difference for Shadow priest mana. But it'll also hurt healing mana a tad, unfortunately.

But yeah, the values are currently reversed.

Yep, just a bug. Should be fixed soon.