at higher boost levels on some gt series turbos they are allegedly a must due to weak/thin turbine shafts and relativley weak plastic bearing cages, but they are not neccesary on t series turbos as they are very strong. i have noticed on a few cars i have owned there has been a little quicker (just noticable) in being "on boost" between gearchanges with a dump valve fitted and just a touch (minute) bit of "lag" without one fitted. but if you have a cone filter the car without dumpvalve sounds far better imo