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I am not sure changing the fuel pressure on the 98+ GS would prove 4-6 rwhp. I can tell you that you will run rich especially on a car that doesn't have a fuel return system and an ECU that has volume specific parameters (monitored by hordes of sensors). Honestly, I think you should avoid using a FPR and go directly to A'Pexi or HKS Super AFR, less restrictive intake and bring it to a respectable tuner. I think you might have better luck flooding your engine with a higher output fuel pump.
A "fuel pressure riser" or and adjustable fuel pressure regulator will not work on the 98+ GS soley for that fact that the fuel system is a returnless system. This means that the fuel pump and fuel pressure regulator is in one assembly in the fuel tank. The fuel pressiure riser/regulators work great for the return systems that have the stock fuel pressure regulator after the fuel rail on the intake manifold.
You will be better off using an electronice fuel adjustment computer to tailor the fuel delivery according to the new air-flow characteristics created by install an intake and exhaust.