ok i've let this shit show go on long enough
cross drilled rotors are for reduction of weight, period
the cars that come stock with drilled rotors also have rotors that are usually 12+ inches in diameter, that makes them weigh a ton, so to make up for this they drill the rotors to remove mass and weight, braking is not sacrificed because the added diameter makes up for the loss in surface area from the drilling
they use to also serve the function of allowing the gasses from a hot brake pad somewhere to go and keep the pad from floating on the rotor on the layer of gasses (similar to hydroplaning) but modern brake materials do not gas out like that
cross drilled replacement rotors in OEM sizes will have less braking force than solid rotors due to the reduction in surface area, contrary to popular opinion they also do not cool off any faster than solid rotors, they are however far more prone to cracking under high stress use, and in most cases will flat out say not for racing or autocross use
slotted rotors are for removing glazing from the surface of the pad (not as important with modern pad materials)
as the slots pass over the pad there is a small amount of pad surface shaved away reducing the possibility of the pad glazing under high temp from repeated hard braking. surface area is also lost with a slotted rotor
cross drilled slotted rotors in OEM sizes are a scam designed to take money from people who either do not know any better, or simply want the appearance of performance (and dont care that they are actually losing braking performance)
they are the equivalent of putting a fake intercooler in front of your radiator that gives the look of performance but in all actuality only serves to block air flow to the radiator reducing the effectiveness of your cooling system
and here is a word from
TireRack.com regarding this very thing
http://www.tirerack.com/brakes/brakes.jsp?make=Brembo&model=Sport+Drilled+Rotors
Note: Brembo Sport drilled brake rotors are not recommended by The
TireRack.com for track use or intended to be used in conjunction with race compound brake pads on the street. For severe duty brake demands see
Brembo Sport slotted brake rotors.
Important Reminder: Slotted, drilled or dimpled rotors offered as OEM replacements should not be considered appropriate for high-speed track use.
While grooved, drilled and slotted rotors offer an enhanced appearance and add some resistance to the boundary layer of gasses that can build up between the pad and rotor, they are not designed to withstand the extreme temperatures that are produced on the racetrack. If they are used on the track, it is very important that the rotors be carefully inspected and should not be driven on if even minor signs of deterioration are seen. Note, too, that if any products are used on the track they are not warrantable.