This is what i read about dumpvalves-----
Although a dump valve in itself will not increase power it reduces the effects of turbo lag between gears. When the turbo has fully spun up and is pushing its maximum boost to the engine when the throttle is closed during a gear change the turbo will start to push against a dead end. This causes the turbo impeller to loose revs and spin down. When the next gear is selected and the throttle again opened the turbo will need to spin up to its boost pressure again. By fitting a dump valve the turbo can keep spinning since the valve is releasing the built up pressure to atmosphere. This hence reduces the lag of the turbocharger and allows the power in the next gear to come in much quicker. Note: During very severe driving and higher boost pressure the dead end effect of a closed throttle can cause the turbo impeller to shatter due to the high pressure it creates.
So i would think they help in someway.