I've had an ebay junk bov on my car for 50k(im not bragging) and my car runs stronger than any other "stock"(intake & CBE) wrx that i know of. Just ask jwelder13.I was going to remove it after being "scared by the mumbo jumbo but i've never had any kind of problem whatsoever.And my car pulls harder to redline. Also the turbo spools faster which means target boost comes quicker. Hence quicker between shifts. The bov will not blow your engine. Remember, most aftermarket part that you put on your car (engine) decreases the amount of total mileage that you would get compared to leaving it stock.Drive it like you stole it and burn that fuel. make sure you have good plugs and do routine (or more than) PM. I have talked to alot of people about this, subaru genius's and nobody's. but when I switch from non to with, I NOTICE the difference and I like it, therefore I keep it. Just my 2cents. -Jake (also i really dont care if my engine blows it just gives me a reason to go with an sti motor)
edit: just noticed you have an 04' they are a little more sensitive than my 03' ecu wise, Good Luck