Your ideal 25-man raid group

[Delc]

True dat. I guess you have to figure out where best gains are from your shadow priest(s), and this may actually vary from fight to fight. But warlocks look to me like a case of obvious synergy since they're the ones that suffer the most on mana, and rely on lifetapping to fix it. A shadow priest's mana restoration will help healers, but generally not the life restoration.

While I certainly wouldn't turn down a shadow priest feeding me health and mana on my lock, I'm not sure how much you would really gain. The biggest limiting factor for locks is usually threat. Soul shatter should help a bit, but with a 5min cooldown it will be pretty easy to push the threat ceiling a second time in a long fight.