From the book "Training WRX" written by MRT performance tuners and engineers:

"Externally venting BOV's can present a false picture of engine operation to the ECU. When an externally venting BOV opens to atmosphere the airflow meter sees a large increase in air flow that is not enterin the engine. This can lead to an excessivly rich mixture, backfiring and generally soggy performance. This is why Subaru vent the BOV back into the air inlet system.
Installers will adjust externally venting BOV's so that they stop venting soon after the throttle is lifted, which alleviates the ECU problem, but this will actualy reduce the BOV's performance.
...
It remains the case that , in any car with less than a full-house monster tune, an after-market BOV is not really necessary. Almost all are fitted to provide the characteristic 'sneeze' or 'whiplash' of an externally-venting BOV."

so the truth of the matter remains unless you're running FAR more than stock boost a BOV will hurt you, unless you get a reprogramable ECU and have it tuned... in which case you're right back were you started at stock and a lot lighter in the wallet.

 

I've been running my BOV for several months now with none of the listed side affects. There are many drag racers that have ran with and with out a externally vented BOV and they all state no changes in 1/4 mile times. I also haven't seen anything in my tuning logs that would indicate any kinds of problems. However, without a wideband AFR setup I can't tell you really how rich you run between shifts. As the thottle body is located well after the MAF sensor, and the throttle is closed durring shifts the ECU already compensates for much of this problem with the stock fuel maps. The 0 percent TPS maps run rather lean in the first place. I would theorize that a BOV would cause this to run either stoic or slightly rich. I haven't seen any evidance that a BOV hurts performance. However, it definately does not help it.

To the person who stated that a BOV is a must on a turbo charged car, you are correct. However, the Subaru comes stock with a air bypass valve which plumbs back into the intake, this ABV is good to about 20 PSI or so. Replacing this with an external venting model is what this debate is about.

 

yeah despite what that says I haven't heard of anyone haveing serious problems. I have heard several people comment on a slight hesitation after shifts after having installed their BOV.

my only problem is people boasting performance gains where there really isn't any.