Valve spring upgrades are a must at higher boost. Subaru valve is 36mm / 1.417" diameter. So it has a surface area of a bit over 1.5 square inch. (π R^2)

Hypothetically, If you put 30psi of boost against the back of the valve, thats going to be 45 pounds of force trying to open the valve. (30 x 1.5) Thats 45 pounds of force the spring has to overcome before it even does anything to controlling the valve during the 4T cycle.

What starts to happen is the valve spring has a hard time controlling the valve, and it no longer follows the cam ramp profile smoothly. And it begins to hammer on the seat and wear or crack the valve.

I dont know how accurate your specs are for OEM spring, but you say 49 pounds force at seat pressure. Thats not very much left over when you take 45pounds of force from the 30psi of intake air pushing against it.

The valve springs I have going into my engine have about 80 pound at seat, and like 220 pound or something at 10 or 11mm lift. Whats important with valves is not so much the poundage, but the rate at which it increases as the spring is compressed. But most valve spring options for Subaru will not be classified this way. Instead they will typically give some force values at seat and a open levels of lift.

And Subaru valves are small, imagine the valve springs needed in a blown V8 with 1.5inch diameter valves...

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