Well, I know it doesn't give you any numbers on a dyno. In a closed atmospheric loop it recycles the pressure back into the system. If you release that pressure it might even lose some HP in the long run. In other turbo systems the "blow-off" is used to cool the turbo which increases power. If its just the sound you are looking for, they sell these on eBay for much much cheaper then the money you just wasted on an aftermarket BOV...
http://cgi.ebay.com/ebaymotors/Fake-...spagenameZWDVW