When you drill holes in rotors (or slotted) you lose effective friction area. Pads are the most important item in the braking system. If you could increase the size of your rotor it would help in your braking because the applied torque to stop the car would be a lot less. But you would either have to go to different calipers or make brackets to move the calipers out.

Last night I took a customer car (92 Honda Accord, yeah a ricer)
out for a spin. He was telling me how his handling sucked, but how great his upgraded brakes were. (Slotted and Drilled standard sized rotors, Metal Master pads, S/S brake lines) After a few turns up the hill I lost his brakes and they overheated!

Oh and i forgot to mention without the proper type of air flow to the rotors the drilled holes are useless. except to look pretty?