You may want to get the specs from you appliance supplier. I have ran across cooktops with a 12.5kw rating on them.
As for your ovens a 8/3 should be fine. But then again I have seen some real doozies by Dacor and Thermador. It's best to check first than repull later.
I always leave enough slack on my built-in oven feeder to pull it to an adjacent cabinet and then fish the appliance whip to a 4insq. Makes it easier to make up and it is much more accessible.
