Well considering hundreds/thousands of Subaru WRX owners have run decapped injectors with out needing an adjustable fuel pressure regulator I don't see why you would need one. Mainly because injector pulse width and latencies along with proper injector scaling produce no more fuel at idle than stock,unmodded injectors. You are putting the cart before the horse. Have you looked at you fuel trims? Have you even verified your fuel pressure with a gauge even? If the stock FPR is one is bad,replace it with another stock one. At sealevel with the reference line removed from vacuum you should see around 43.5 psi. At idle fuel pressure should be between 35-38 psi.