The fitting of non genuine parts, including non 'N' rated tyres will only affect a Porsche warranty, new or used, if the part caused the problem. You will not be refused warranty cover on engine, transmission etc if you have non 'N' rated tyres or a non genuine battery fitted for example. Renewal of the warranty is different, then any non genuine parts do matter for some strange reason but I don't think the dealer will worry about inspecting your discs or pads.