I had to turn down the velocity after installing a Hush bolt, seems that the theory of sealing the breach is sound and wastes less gas by channeling this through the bolt.
Can't really comment on sound difference as I'm too busy shooting to be worried about that
The lastest style of bolt is made in a newer delrin and should be black inn colour, also it has 'pooty' style cuts to the body to help reduce bolt rub, there's also a new soft face bolt called the Kush bolt as well