I'm not convinced its an "upgrade" swapping single pot for single pot
The S3 setup is a very noticeable upgrade from the stock gti/gtd system...
Yes they may still be single piston sliding calipers but it's a big piston (51mm) acting on a much larger disc (345mm versus 312mm),
With a 4 pot system you are buying lightness, endurance and 'feel' over a basic OE single piston system, even with £2k worth of AP 4 pots you are still moving the same amount of fluid via the master cylinder, you just get more of a progressive 'feel' due to the staggered way in which the smaller and larger pistons operate

The S3 setup is very capable especially when running uprated pads and the weight can be offset by using 340mm TTs discs..