It is not good to compare exhaust pipe diameters of normally aspirated and forced induction engines. There can be a lot of debate about this topic, but it all boils down to back pressure at the end of the day. There is a magic number for back pressure, but since it's rarely measured you can assume a certain diameter will work well from past experience.
I would use the following diamaters:
G40 with standard or slightly modified engine. 50.8mm or 2"
G40 with many modifications including charger, 65mm pulley, cylinder head, camshaft and increased capacity. 57mm or 2.25"
G40 or Polo engine with Ko3 turbo. 63mm or 2.5"
The reason why you use a larger bore on a turbo is there needs to be a greater pressure difference across the tubine. Hence reducing the back pressure as much as possible with a 2.5" downpipe and exhaust system.