Originally posted by 89TwinTurboSupra
yes it does, if you have no blow off valve you experience compression surge, which will not alllow you to run any decent amount of boost, and which will damabge your turbo. Therefore it does help your performance.
Upgrading from a stock BOV also increases your performance, because stock BOV's will tend to leak at a certain amount of boost, by replacing it with a stronger one, you can increase your boost, which makes you faster, which is a PERFORMANCE GAIN. 
You will NEVER find a PERFORMANCE gain from ANY BOV..... if you ever do please let all of us know....
Read up:
http://jekylandhyde.madpowaz.com/bov.htm