I think the conditions in which the car operates in will have a major effect (think wet and salt).
I agree with John but I don't think discs or pads have to be hot. Some of the fade he describes is what is termed 'speed sensitivity'. Even discs and pads at ambient temperature will gas at speed when hard braking is called for. If a car does not have a sports brake set up, it is usually impossible to lock the wheels at high speed (say 80mph or above) yet the same pedal pressure can easily lock the wheels at speeds below 50mph. The point is pads can gas even when quite cool and this speed sensitivity is something that machined discs and more sporting pad compounds attempt to reduce.
In short, I think machined discs could improve high speed braking even without using a sport pad (which tends to have trade-offs in cold response, noise and disc life). I'm sure different drivers will have different experiences based on variables of choices, their preferences and operating conditions. It's a bit like tyres in that respect but I don't question an individual's experiences or the value of such.