leaning out adds hp at different engine speeds. that is why CAI show numbers on dyno's. If you don't believe me. "Search" and it's also bad for your engine as you said.

 

I'm not trying to say that you couldnt hear the bov on your car but I also had this mod for around 40k and never heard the bov and barely heard the turbo. After finishing my TBE off with a TXS DP and flashing Cobb stage 2 the silencer mod did nothing but make a big sucking sound so I reinstalled the silencer and I am much happier with how my car sounds.

I would think the filter could play a role how much you hear the BOV and turbo. I use a STI pannel filter what do you use?