The only problem with KM is the -damage isn't up all the time, only when Power Siphon is up.
As for primary, ice does more -damage than shield does, and it also does the -recharge which would be nice for team play.
I had thought about doing the same thing, stacking -damage, but when I realized the KM -damage isn't up all the time, I changed my idea to a Dark/DP defender running chemical rounds.
I had forgotten that ice did -damage in Chilling Embrace, it might be worth trying again.