I had a boost leak test done on the car very recently which showed no leaks whatsoever. I do hear a good deal of wastegate flutter on acceleration, but I was under the impression that sound is normal.
No fitment issues when installing the downpipe...is there any possibility that the leak may only appear at high pressure (i.e. peak boost) and not at the pressure level that was tested? From searching, it seems like generally people lean toward blaming the BPV for noises like the one in the video.