search for BOV on this site to find a ton of information. To summarize:
-BOVs don't give any performance gains and can damage your car if improperly installed (which is also easy to do)
-The stock BOV is good for more PSI than your stock turbo so you don't need to replace it
-the stock BOV is a 100% recirculating BOV (as required by law) so the air it expels goes into the intake. An "atmospheric" BOV doesn't do this
If you want to hear the BOV, any mod that removes the intake silencer will suffice. If you want your BOV to sound like a gun going off, a) you're a ricer b) buy a TXS RFL (really f-ing loud) BOV.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD