Use the rotors until they fall out of spec. The won't just quit working. Don't let them go to far out, but your car won't know if they exceed wear limits by a little. And you do not need to replace pads and rotors at the same time. A Porsche is no different than any other car in that respect.
FWIW, new rotors are like $125 for a set (either front or back). You don't need oem. It's easy to replace them yourself without any prior experience. So, if you feel a little on the cautious side, there's not a huge financial burden.
-td
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