Just flicking the switch on Mind Blast's haste cooldown reduction (was in for, what, one build on beta?) would be a very decent start, and would actually do more for Shadow than giving Archangel's +25% Mind Flay damage back (which current PTR build Insanity does +100% mind flay damage, go figure).
Say it with me now guys, WE DO NOT WANT TO BE RET PALADINS. Say it again, come on. WE DO NOT WANT TO BE RET PALADINS. They have a lot of cooldowns, and because of it, when their cooldowns aren't up, their damage suffers. It's not fun to only be able to feel awesome for 18-25 seconds every few minutes, and suck the rest of the time. We need our baseline damage issues fixed first.