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..
I'm just a tad confused when you intially said 345 then 340 at the end will this not put the caliper out 5mm? Pity they don't look as awesome as the 4 pots :-(
The 345mm disc fitted as standard leaves a lip on the edge of the disc when worn (2mm ish), the TTs disc doesn't have this lip as the pad runs right to the edge (the TTs disc at 340mm is only 2.5mm smaller where the caliper position is concerned - radius not diameter is the key factor)
The TTs disc as Andy says also benefits from having much better cooling vanes and is over 1kg lighter than the 345mm version
If you are doing the upgrade for mostly cosmetic reasons then stick with the TTrs setup you have..
