There are multiple 1.8t, 20V VW engines in these cars. You can find your specific type by the three letter code marked on the head (AWM, AEB, AWT, etc). I don't know if the blowoff valves varied since I think they all take the same turbo so I would just go with the stock unit unless you have upgraded the turbo. There is no performance to be gained by changing this since its only function is to provide a path for unwanted boost to be released.
"Dump" or "Blow-off" valves are not really compatible with these engines, and not recommended. They use instead recirculation valves, which re-route unused air back into the engine. Since this air was already metered, venting it will inevitably throw off your fuel/air mixture.
If you really want the "psssst!" sound, some companies offer combination valves, which vent a little of the air for sound effects, while still keeping fueling mostly correct.
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