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I would never drill a rotor that wasn't designed from the manufacturer to be drilled. A Brembo rotor on a Porsche that's drilled was part of entire brake package that was designed for the car. And the rotor was produced to be drilled.
Drilling a stock rotor is dangerous and does nothing to improve braking. Think about it. When you drill holes, you've removed 20-25% of the friction surface that the pad contacts. So on a street car that doesn't build up the heat of a race car, you can give up brake performance in a big way.
Also, having talked with a Brembo engineer a number of years ago, he also said that someone who drills their front rotors but leaves the rear alone is messing up the brake balance and the ABS. He said that with a front that has 20% less surface friction area, the driver has to apply more pedal pressure to get the same braking force from the front than they did with the stock rotors. But, the rears get the same increase in pedal pressure and still maintain the stock friction surface. So in the wet, you know have a vehicle that's going engage the rear ABS before the front, which isn't the way the ABS system was programmed. It'll work in almost all conditions, but as he said, do you really want to test your homemade brake balance in a panic situation when your life depends on it?
Oh, and I've seen stock rotors that have been drilled ultimately crack. I foolishly went against my own best judgment and decided to try a set of 4 drilled Zimmerman rotors for my 911 a couple of years ago in an effort to find a competitive advantage. Keep in mind Zimmerman makes the stock factory rotors for the earlier 911's so these aren't no-name brand rotors being drilled. Here's what happened after just 8 track days. And when I pulled the other front one off, there were cracks forming too. Note that the big crack followed a line from hole to hole. And note the cracks around every other hole migrating outward. This crack was all the way through the rotor. I threw the stock undrilled ones back on, my brake temps went down over 100 degrees, and I could go deeper into each turn before standing on the brakes. Drilled stock size rotors took away brake performance, increased brake temps significanty, and could have caused a major accident if the rotor had cracked at a part of the track other than a slow speed turn. Oh, and when it broke, it sounded like a sledge hammer hit the right front wheel well it was that loud. If I'd been hard on the brakes just one turn earlier (VIR, 140 into turn 1, a 2nd gear right hander) rather than T3 which just requires a light tap on the brakes to scrub just a few mph, I would have locked that right front tire and at least gone straight off into the field, or worse, could have spun the car at triple digit speeds.
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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R
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