Knowledge (Undead)?

[Lord Pendragon]

Would a +2 synergy bonus to turning damage for 5 ranks in Knowledge Undead be too high?

Yes. Clerics already are really strong turners, and most undead of appropriate CR have less than even odds of resisting the turning attempt. Adding an additional +2 on top of the +2 they've already added for Knowledge (Religion) is most likely too much.

I'd go with what the Jester suggested. If you and your DM want to keep Knowledge (Undead) as a separate skill, move the turning bonus from (Religion) to (Undead), but do not introduce a second bonus.