I just got my tial 50mm BOV (Q series) to start working correctly. Right now, I am running 10lbs of boost on my GN (new build). At 10lbs of boost I can hear it working really well. I don't hear any more turbo flutter when i get off the gas. At low boost it works good also, you can hear the BOV open, but it is not a loud swoosh, only a suddle one. Even at 10lbs of boost, the swoosh noise isn't loud, u can hear it but, it isn't as loud as i have heard some people complain about. I played with the spring for a couple weeks and finally was instructed to use shims. At idle, warmed up, I was reading 15-16Hg/in on my guage which matched up with a plain spring (10lb). This spring was too hard. It would keep the BOV from opening at idle, but it was too stiff and didn't allow the BOV to open once the petal was lifted ( and I was still getting turbo surge/ flutter). I contacted tial and they sent me a 9lb spring and the spring was soft enough to allow the BOV to open well once pedal was lifted, but the BOV would fluctuate open and close during idle. So, I used 3 (1/8in thick) washers as spacers and it works fine. At idle, it is not as responsive as the import videos on YouTube (plus it only opens up about 3/4 of the way when testing at idle). Under boost, I can't see how much it is opening, but I don't hear the turbo flutter (most important) and the BOV sounds much better than what it was with the 10lb spring.
With 15Hg/in on the vacuum gauge using a 9lb spring with 3 (1/8in) washers keeps the BOV closed during idle, and keeps the BOV reacting correctly with no turbo surge under boost. Of course I would've preferred no shims, but there is no "in between" spring between a 9 and 10lb spring.