I've never driven any brakes that work better than the Brembo upgrade kit for the S3 - the GT or GT Junior.
The Porsche brakes look more bling, lighten your wallet better, but work no better.
2pc discs on the road can be a pain too...noisy, risk corroding and sticking.
Porsche went away from them for a reason...(the 996 Turbo has single piece discs)
Save your cash and buy the Brembo kit - the same as the SEAT Leon Cupra R kit.
It's designed for the master cylinder fitted to the S3/A3 so you keep the feel, and it doesn't throw the brake balance out.
At the end of the day, Porsche don't make their brake callipers, Brembo do.
If you want to pay lots of cash for a Porsche badge in thinking it'll give you better brakes than a similar calliper with a brembo sticker on it - go for your life.
...a fool and their money, and all that.