I have my fuel pressure set high since the computer can't open the fuel injectors long enough to provide the amount of fuel I need at WOT. Injectors must open and close so they cannot possibly run at 100% duty cycle. If they only can run at 90% duty cycle, and I think that number may be lower, that means that 24# injectors can only realisticly supply 21.6# of fuel in an hour at 39psi. That's enough for about 346hp( Assuming .5 #/hr/hp). By increasing pressure, more fuel can be delivered per injector pulse. At 47psi the injectors essentially become 29# injectors able to support about 417 hp at a 90% duty cycle.
As a side note, people have to make sure that their fuel pump goes with their injectors. A liter of fuel weighs about 1.5625#s. Multiply your pump capacity times that number and then divide by 8 to figure out what size injectors it'll support.
As for improving after each fuel pressure tweak on the dyno or at the track, there's still the fact that the computer hasn't adapted yet. It may be that the O2 sensors are reporting a leaner condition that really exists and moving fuel pressure down compensates fot that for a while. Then the computer adapts again and you're back to running rich. I know this from experience. When I bumped up my pressure it ran a lot harder throughout the powerband. That power is gone now since it has adapted.
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351W 89 Mustang GT Convertible