it's not odd that it's reaching 12psi and staying there at all, your system is obviously no longer airtight above 12psi, the hissing you're hearing is the turbo trying to make more boost, and it's all leaking out at a steady rate from somewhere! it's pretty logical that a leak would give you stable, but reduced boost.
Unplugging the N75 electrically isn't dangerous at all, the N75 valve default position when not powered is 100% open, so it allows free flow from the charge pipe to the actuator, so by running the car with the N75 unplugged, you're just running actuator pressure, and reduced power.
I've actually got a switch wired into my N75 wires so I can switch between actuator pressure for normal use or full boost for fun.
On a ko3s setup unplugging the N75 will take you back to around 150-160bhp. On my hybrid setup actuator pressure is 274bhp, and full boost is around 300, so it's a measurable difference.
