Them discs, original or not, are not stainless, you can see the casting marks in the holes, they have internal venting, it's impossible to machine that. They are cast iron.
The brake pads also have a high iron content, now think about it, what's likely to break down under high temperature and pressure, the 20kg cast disc or the 200gram pad that can't dissipate the heat.
I used similar (hugely recommended etc etc) high iron content pads in the past on oem discs on another car and my discs cracked in half. Tried uprated discs etc etc in the end, different pads solved it and the braking was better and more consistent. Some pads just generate much more heat than others with similar performance, once they get hot the braking force drops off and you end up killing the discs.