After reading the adjustable FPR debate, I have to side with Willwren.
If you read his posts you have to. He was running lean an IDLE AND AT WOT. So, lean ALL THE TIME. Obviously, the only way to increase fuel and bring the A/F ratio back to normal is to either change to a larger lb/hour injector, or increase fuel pressure. Increasing the pressure to an injector will increase the amount of fuel it sprays during it'* on time; this on time, or "pulse width" is the only control the computer has over fuel delivery. When the computer has to increase pulse width over a predetermined amount, fuel trim values increase. So if you need more fuel, you can increase pressure and the computer therefore won't need to increase pulse width (much) to achieve higher fuel delivery, meaning fuel trims will remain near zero.
I see where people who dissagree with Wilwren are coming from. They're saying that if you're running lean up top and you increase regulator base pressure, you will run rich up top. Ignoring the fact that Wilwren is RUNNING LEAN ALL THE TIME, NOT JUST UP TOP, they are correct. If you were running lean up top and not down low and you increased base pressure you WOULD run rich down low until the computer compensated, and then this would be reflected in the fuel trims at low RPM, as was said. Remember, the computer stores seperate fuel trims for different engine RPM and load conditions. However, I don't see why the change in fuel trims would be bad. It'* a necessary side effect of adding air. If you go to a larger lb/hour injector, you'll still see the same change in fuel trims. That'* assuming you don't reprogram the PCM to recognize the larger injectors anyhow.