The main difference between EX Haku and CP Haku is the scope of their goodness. By that I mean, EX Haku is just great across the board. CP Haku, on the other hand, is great in some specific areas; his advantages are focused on a point, like a knife.
Most of the time he isn't going to be able to take advantage of the OD 9k combo but it's just there for those "oops I win" moments.
It's obviously ridiculously strong but I'm not sure where he would be if he didn't have that since otherwise he wouldn't have as much explosive power. It's not something you factor into the average match but just the fact that it's there can mean something when gauging the character's strength.
(Also someone mentioned Tao in CT. She ended up around A tier by the end of the game's lifespan, IIRC.)