Stock STi's run PIG rich. I'm talking less than 10:1 at WOT. If you were to run atmospheric on the BOV you're going to go even richer in between shifts.
At the worst I'd run 50/50. You won't hear it much at all if you put it 100 recirc unless you do the intake silencer hack or just buy a cheap intake like the GPMoto.
Intakes work great on STi's because they lean out the car a little bit (still keeps it safe) and makes good power plus makes the turbo sound very good!