A lot of debate about this, the air pulses are getting bounced around all over the place with one end trying to shove the air and the other saying nope I'm staying shut, so the noise you hear is the impeller getting battered by pressure waves.
In short most agree a small amount not really and issue, a large amount of boost is bad, and will shorten the life of the turbo, by how much is like how long is a piece of string!!
It will by it's nature induce some lag, as the pressure waves slow the compressor down, whereas releasing the pressure with a dump valve allows the turbo to stay spinning faster, and so be on boost sooner.
If you want to have both simply use a mondeo front heated windscreen air valve wired via a switch so when open the dv will work as normal from the inlet vacuum signal, when shut(OFF) you will get no vac signal as pipe is effectively blocked by screen valve so you get pressure back up, ie chatter, you won't get the full amount as the pressure will overcome the spring holding the DV shut, so some pressure will come out of here, still get a lot though, to test simply remove vac pipe from DV and plug the pipe with 6mm bolt etc then go for a drive.
tabetha