Turbos don't need backpressure.
The 6-7psi opening is useful because otherwise it would just be a loud cutout. These things are made to try to have something that has a streetable sound level and more power under full throttle/boost.
Here's one for the Evo X:
https://englishracing.net/products/e...ut-evolution-x
Make your own, get a wastegate and stuff it where you can.