I'd check the condtion of your DV first to make sure there are no splits in the diaphragm, mine was shot and you could hear a sound like air escaping from around 3k rpm upwards, it affects the performance massively too, I read an old copy of Volkwagen Driver mag the other day and a chap who had his mk5 GTI on the dyno was only making 160bhp, turned out it was down to a torn DV!