Stock M157 system does not come with any blow off system. When stepping off throttle, all the excess boost is recirculated bank into the turbos, slowing down the turbo wheels. This results premature wear of the turbines.
 
We have put a kit together using a mechanical BOV operating on vacuum that vents the excess boost into the atmosphere. Not only does this protect turbos, especially under tuned and boosted application, but it also sounds like a proper turbo car now.