Slotted rotors are supposed to keep the pads from glazing, by exposing a fresh surface of friction material on the pad when the brakes are applied. Basically, the slots supposedly cut into the pads (just a little).
Doug Lewis of FPS offers slotted Brembo rotors for a good price. Brembos seem to have the most mass to them, which is a good thing. More mass = a tighter grain in the casting, higher heat dissipation, and less chance of cracking at the slots.
TireRack, and many other places, clearly state that slotted, dimpled, drilled, anything other than solid rotors, are not to be used for track events.
For the rear upgrade, do you mean vented discs? If so, there's very little selection of good pads for the vented rear setups. At least nothing I could find, perhaps someone else has other info. The solid setup of the 93-99 cars has a very wide selection of pads, including PFC carbon metallics and Hawk HP's.
If you meant the larger 11.6 rotors on the rear, IMO, it's an aesthetic upgrade. The same size pads are used that were on the smaller setup, so swept area remains the same. The radius of the rotor increases by .75", which in theory would give you more braking power (torque=force x distance). I've driven cars with this setup and bias plugs, and they didn't really feel all that different than my 91 with the 96 brakes, stock rears, and bias plugs.
