well on the fiesta XR2t running the T3 hybrid I was running 28/30 psi
and the Skyline it's 1.2 bar about 16-17 psi both have been fine but both have been uprated,
You can get an adjustable dump valve so it flutters under a certain boost and dumps over that spring rate setting,
best of both worlds

As for the difference between running and not running one, it's slightly different on the way the turbo comes in and the delay of engine breaking, the dump valve the boost dumps and is really sharp responsive when you accelerate and you get better engine braking,
without it's slightly smoother on and off boost and it's better if coming of and back on the accelerator quickly once on boost but running higher boost levels the engine takes slightly longer to get rid of the boost, but it's only just noticeable.
The best thing to do is buy a dump valve blanking plug and experiment lol

I like both but
the dump valve wins really due to turbo longevity and It's a track and show car so has to make a noise lol
Matt