So you do have a spring? I see....
You understand the grainger valve correctly. It prevents any pressure in the line until a certain pressure is met, then it lets that signal through. The dodge guys set this check valve to what ever boost setting they want to achieve. The valve is adjustable. it takes a little practice, but you can set it anywhere from 7-30 psi (or more).
It was designed at first to be applied to stock cars. The stock wastegate spring is for 7 psi, but starts creeping open at 0 psi, as most wastegates do. Instead of slowly creeping open from 0-7 psi, they want the valve to slam open. They want it to stay shut until 7 psi (or what ever setting) is met, and then open quickly to stop boost creep. The idea is that boost comes on much quicker. I've ridden in a car before and after this modification, and the results are tremendous.
From what you're saying, I still don't see how you get the same effect, as you said yourself you need to set VEMS at 160 kpa if you want to see 210 kpa. That just isn't right. =]