Well you need so many cards for the overhead panel because every mastercard can handle 72 inputs and 45 outputs.
If I'm correct, you already need about 100 outputs to control all annunciators and about 150 inputs for all the switches so that makes 3 mastercards to connect all these controls.
Just check the sioc script from Georg that you can download here:http://www.flightdecksoftware.com/files/ohlogic-beta04.zip
You can see how many inputs and outputs you need.
I don't know anything about the cards from CP Flight so I can't tell you more about that.