ytry, do you have a WB in your car? I'm wondering if your afr is really rich on decel? it seems like your bov is still not sensetive enough and when you close the throttle it's still drawing fuel from the boosters. with that said I'm wondering where you have the boost/vac signal to your afpr at??? this might be a problem also.
The purposes of the bov are to keep the comp. wheel from slowing down to much between shifts (on manual trans only [obviously]) and avoid a surge in the charge tubes that might cause the shaft to shutter and ruin the bushings. it's virtualy impossible to spind a turbo in reverse. When you don't have a bov or it is set to tight you here that che che che che sound, imagine the shuttering that wheel and shaft (that btw are spinning at about 100,000rpm) are encountering
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ytry you might consider a supercharger baypass valve like mentioned earlier by blown353.
just my .o2 worth, don't spend it all in one place