For the record, VAG 1.8t engines use a diverter valve rather than a blow off valve for a reason... the charge air has already been metered by the MAF and the ECU has made its calculation for fuel based on the airflow etc... when you let off the throttle, the charge air pressure is released through the diverter valve back into the intake system after the MAF as the ECU is already expecting to see this airflow...
Using a split R bleeds some of this measured air out of the charge air to make the 'noise' which results in the mixture running richer than it should... this can lead to the engine running erratically
You would get more 'whoosssssh' from a 007p/panel filter combo but even then as I said before, there isn't enough boost pressure on a std engine to make much if any noise either way...
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