It's not an "upgrade" for a turbo, it's necessary. When the throttle plate closes during a shift or if you lift off the throttle while driving, the compressed air needs to go somewhere. If you don't have a blowoff valve, the air tries to force its way back into the turbo, which is, umm, a bad thing. On most stock turboed cars (WRX's, older Eclipses, etc), the air gets plumbed back into the intake manifold rather than into the atmosphere. A full or partial atmospheric venting blowoff valve is what makes the squeeking or whoosing noise. Or, if you remove various stock silencing mechanisms on a stock car (like mine), you can still hear a full plumb-back BOV.