you have enough fuel pressure, but your low WOT RPM might be a lack of fuel volume....that could be a sticking anti-siphon valve on the tank, or a pick up tube that has crud getting pulled against the screen at the bottom of it. Any inboard I think is going to have an anti siphon valve unless someone removed it, I would think about checking that, because it could be your problem.
think about it like this, to idle and run at low rpm takes relatively little fuel in terms of gallons per hr., but to plane the boat will take between 2-3x as much in terms of gallons per hr.
My boat had this problem when I first bought it back in 2002, a good mechanic found it and after that was fixed it ran well for years. I do think that these parts should be inspected on a regular basis, because it doesn't take much of a restriction and you're having this problem even if everything else is perfect.
