I'm new to WRX's but definitely not new to turbo charging. the reason that there is slightly more lag with ATMO is because the turbo has to work to get the air through the intake all over again.Going from fully atmo to fully recirc you can feel a difference in lag after shift. I wish I could quantify it for you but I can't. My buddy couldn't tell the difference from one to the other, but I could. I felt the difference, however miniscule it was.
Now, if you take the air that the turbo just compressed, shoot it out of the BOV right into the intake plumbing BETWEEN the air filter and the turbo, then you just forced the air to go two directions - one is out through the intake, and the other is right back into the turbo - causing positive pressure in the intake piping. The air going back into the turbo allows it to spool much easier due to the intake pressure caused by the BOV. If it is vented to ATMO then there is no pressure in the intake piping, but there is in fact a vacuum - and the turbo must work harder in a vacuum to get boost.
hope thats clear.