...you just need to ask what they do. They provide a big heat sink and big heat-dissipating surface area. You don't need to manage heat on a short little autoX course. Period. Why spend a bunch of money you don't have to? If "people" say you should have them, have them pay for it.
The other thing big brake kits generally do is provide a lot of bite when you use the brakes. This is because of the increased leverage. This can be nice, but you accomplish much the same thing by using good brake pads. For ultimate bite, use a race or near-race brake pad. Watch out though, they will be very abrasive on your rotors at autoX temperatures (low temp).