Since I had an 04 WRX and used the same XS manual cheapo boost controller you're talking about, I can tell you that 14-15 psi is where you will safely max out at. Just don't floor it in 5th gear going 60, or the boost will spike to 18 or so and you'll get a real violent fuel cutoff.
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2000 FRC: Custom X-pipe w/ Corsa Pace exhaust, K&N CAI, Black Factory Z06 Wheels on Potenza RE050s
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