Knifing can be one hit, but I am not sure if it was just on a injured person, I think you have to aim for their head. It was needlessly nerfed though and I hope they will readjust it.
I got my platinum star on my UMP-45, maybe it's finally time to get a new weapon.
I am still waiting for them to come out with a real map pack, although I am guessing it's not going to be free.