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I wouldn't run more than 2.5", maybe 2.75" on a N/A... well, unless it is a stepped design. A N/A MKIII doesn't flow enough air to ever make use of that much space and in the time it takes to fill it the gasses have cooled far more than you want... and on the other end of the equation you create a scenario where the vehicle has to push even harder to clear the exhaust path costing you power and responsiveness.... much like the EGR system, the losses are negligible but the added noise is stupid.

The ideal setup for a N/A is a system which gradually increases the further the system gets from the engine. The headers start out small creating a great deal of velocity and promoting scavenging (due to suction from the gas' inertia), then slowly increasing size to allow for the cooling air to continue to move at it's constantly reducing speed preventing the newer gasses from backing up behind that which has cooled and slowed ahead of it. As it's difficult to make a full exhaust in the shape of a massive cone you'll find some high end systems use small increases in the pipe diameter as the system progresses toward the outlet... I don't know of any production systems like this though. Another thing you can do to help is insulating the piping via heat shields and ceramic coating.