Just an added note...
a BOV, used in the generic term is often mistaken for something it is not.
There are different types of blow off valves...Recirculating units, that obviously recirculate the pressure blown off back into the intake charge, 100% atmospheric BOV...vents to atmosphere and gives a sound, and hybrid units that do both (recirculate at lower pressure, and blow off at higher boost levels).
In most cases, recirculating valves are the preferred, for their ability to hold boost, build boost quicker after shifts by not losing pressure, no extreme rich conditions after shifts, and if they do happen to leak just a little, you'll have minimal negative effects. Atmospheric BOV are mostly "cosmetic" in nature...the WRX and Sti use a Recirculating valve stock...in most cases it's good up to 20+psi also (for the STi version, or mildly modded WRX one...since they did use different BOVs).
Just figured I'd add that...hope it helps