It seems that most perf cars have vented rotors all the way around, heck, most cars that have 4 wheel disc have em', so does anyone know why this is?

 

Its over kill for a car to have vented disks on the back wheels of a car. More than 80% of your braking come from the front wheels of the car just as in a motorcycle, where the rear disk is only half the diameter of the front. I believe that if you were racing the car it would be a little different and some cooling holes might serve a purpose in the length of a race. Just like all the vendors on e-bay selling drilled disks... its all for looks and money, or the factory would do it that way.

 

I understand what your sayin', but why is that some "regular" cars have vented rear disc brakes then. My old car, 2000 Contour SVT had em'.

 

I used to have a road-going version of a couple of rally cars. They have vented discs all round. Discs at the back hardly ever got used. More surface; more grip = better stopping from the back (provided not too hot).

I think one of the XF better points is the brakes. Guys at work with Boxster S and Subaru WRC can't believe how quick I can stop.