You can only "lean out" your fuel on the top end. Your computer makes the judgement on how much fuel you use and the only way to change that is with a chip, a programmable computer, or something like the SVO Extender. By reducing fuel pressure you will only be denying the computer how much fuel it can deliver on the high end by reducing the capacity of your injectors.
Some people will claim that they changed pressure, put it on the dyno, and then there was a big difference. The claim is true, but the problem is that the computer hasn't adapted yet. Drive a couple hundred miles, put it back on the dyno, and those number will be gone. At this point, the computer has stored new parameters to adapt to the new fuel pressure. Those dyno numbers will probably never be back, either, since the parameters it was using at the time were based on old samples.
If you, or anyone else, wants to be able to duplicate results received during the dyno, you would have to reset your computer just before doing the run. Find the best pressure for performance and then leave it there. Your computer will adapt, but the next time you race, you can reset your computer and the parameters will be back to what they were when you did your best run. This should last for a few races before it starts to adapt again.
A lot of people tend to think that these computer-controlled EFI cars can be tweaked like they are non-computer-controlled carbureted cars. We have MAF, ACT, ECT, O2, etc. sensors for the specific purpose of determining how much fuel should be metered out by the computer at different RPMs. People are nuts to think that they can tune their carburetor any better than the computer. It's like saying that you can pump your brakes better than anti-lock brakes can.
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351W 89 Mustang GT Convertible