of course it does.. you retain the velocity of the turbine in the right direction, rather than the air coming back and slamming it back the other way....

The way you should have worded it is.... There is no performance benefit of a Blow off valve over a bypass valve.. because they both work to keep the turbine spinning rightwise between shifts and during peroids where the throttle is stabbed constantly, aka in a corner trying to balance the car, drifting, so on..