TBH a DV slows your car down
Every time it dumps, it empties all the boost pipes which must be filled again on throttle up before the boost can kick in - ie, a DV increases lag between gears.
However, it does protect the turbo. Once you shut the throttle, the turbo is still spinning, and still rams air at the closed throttle housing. Having no where to go, it goes back through the turbo and out through the air filter - causing the turbo to stall and puts a lot of strain on the bearings of the turbo.
For this reason, rally teams and seriously tuned cars run "Anti Lag" rather than a DV