[MShift]

ah partial throttle... its normal for a bpv on a supercharged car to vent at partial throttle, but it is generally routed back into the manifold. As for the metered air, ESS uses the factory ECU and sensors with a tune to adjust for the additional air. I'd imagine venting to atm is throwing the AFR off, but maybe not enough to cause an issue or the ecu is smart enough to make up for it by removing fuel. Those would be my guesses as to why VF said it doesnt matter. Personally, I'd use it the way you're supposed to and plumb it back in.

With the number of supercharged cars on this forum, I'm surprised no one with real knowledge has chimed in.

Edit: one last thing...if what you're looking for is the bov noise while minimizing these issues.. there are some options out there that do both and are adjustable. They will recirc at partial throttle, but will vent to atm under hard vac. Might be able to get rid of your whistling on partial while still getting your bov noises on shifts.