Sean, I think you're confusing what the engine
wants with what the fuel pump can
supply. You're correct that at high boost, WOT, the engine wants the most fuel flow, and at the highest pressure. Unfortunately, this is just the inverse of what fuel pumps can provide. They provide the most flow, at the lowest head (pressure). It's just the nature of the beast.
Nearly all pumps (except for piston pumps) have a characteristic curve that shows flow dropping off with increased head (pressure). So unless you have a variable speed pump controller, you can get high flow at low pressure, or low flow at high pressure out of most pumps.
The trick is to know how much flow you need, at what pressure, and make sure your pump is man enough to do it, and has enough reserve to overcome any pressure drops along the way (like smallish lines, fittings, etc.) That's why these pump "dyno" charts are useful.
Walbro's are not a bad pump, however they're noisy compared to the Supra pump, and they've been known to die prematurely, whereas I've never known a Supra pump to die, unless it was 15 years old. There's a reason the Denso pumps are double the price of the Walbro, and it's not just because of the Toyota name on them. For my money, I wouldn't put anything but a Supra or a Bosch 044 pump on a high HP, in tank, boosted application.
You should size/pick the pump with at least 25% more capacity than you need, which will allow for aging and wear, and of course, remember that we run a return type fuel system, so in order for the whole fuel system to work properly, you have to oversize the supply "somewhat". Some people do this with multiple pumps, I prefer to do it with a variable speed controller, like the Kenne Bell BAP.
Also remember that as you pressurise the fuel it heats up, then when you run it up through the fuel rails on the hot engine, it picks up more heat. As fuel heats up, it loses density and energy (this is why old drag racers used ice packs for fuel coolers.) So think about this before putting on a monster fuel system that will pump 1000 HP of fuel, when your engine is only wanting 50-75 HP at cruise. All you're doing with that monster fuel system is recirculating and heating a lot of fuel, so you're going to evaporate a lot of it due to the heating, plus your engine will actually be down on power due to its loss of density. (There are some very good engineering reasons why Toyota and the other OEM's reduce the voltage to the fuel pumps at idle and at cruise; people who bypass this function and run full voltage to the pump(s) all the time are missing this very important point.)
Another problem with monster fuel systems is the people tend to forget the return line. When you're pumping 1000 HP of fuel from the tank, but the engine is only using 100 HP, the rest has to go through the return to the tank. Now, for the fuel pressure regulator to work properly, the return
must be running at atmospheric pressure, but obviously if you're trying to stuff 900 HP worth of fuel into it, there's going to be some restriction, and this restriction downstream of the fuel pressure regulator is going to cause it to do strange things upstream, like raising the fuel pressure much higher than it should at idle and cruise. So then you adjust the fuel pressure down at idle, but take the car out and do a WOT run and you lose fuel pressure and go lean and wonder why?
So IMO, there are just way too many potential problems with these monster fuel systems to use them for a street car. I much prefer designing the system for where it's going to operate most of the time, and using the BAP to boost it for those "occasional" WOT runs.
In my single Denso pump fuel system (which has been running with rock solid reliability on my single turbo Supra for the last six years) I PWM the voltage to the fuel pump down to 8.5 volts at idle and cruise, and then raise it to 12v for low/medium boost, and then run it up to 15-16 volts for high boost. This corrects the problem of the pump's characteristic curve, and matches its output to the needs of the engine. (BTW, if you reduce the voltage to the Walbro, you must use the PWM method, and not just a dropping resistor. We found back in 2002 that the Walbro's will not operate reliably at a
constant voltage of 8.5 to 9.0 volts, but will with a 12v supply, PWM'd to 8.5v)
The key is to try to flow just enough fuel to maintain the correct (42psi) fuel pressure across the injectors under all conditions (and this is what returnless fuel systems do). This way, the ECU stays happy because the injector deliveries stay linear and the engine is much easier to map. When the tuner is finished, the fuel map should then resemble the engine's torque curve, instead of a topographic map of the Rocky Mountains
which you can get when the fuel system isn't working right, and you try to compensate for it in the ECU.
Sorry for the length of this, but oversized fuel systems, and the problems they can cause tend to get me on my soapbox.
