I kinda am. Pressure pumps are pressure multipliers. You put a liquid of one pressure at the entrance and it outputs at an increased pressure on the other end according to it's multiplication factor. From what I've seen, they don't really care what pressure they are fed with.
But maybe this increasing returns only holds true for compressible fluids? I assume gasoline is not compressible.
This doesn't get into flow, only pressure, but overall maximum flow at least increases with dual pump configurations and increases with pressure, and dual pumps in series can sustain higher pressures.
I only have direct experience with and real world data for the BMW e30 configuration which comprises of a feeder pump in the tank capable of a tested maximum of about 190 WHP on its own followed by a similar in-line fuel pump capable of about 220 WHP on it's own.
Together they have supported 400 WHP. This is all on stock base fuel pressure.
I didn't say the combination of the Walbro + Bosch would be a
great idea for 900+ WHP but it's a best guess from what I know about pumps and pump configurations. I wouldn't be surprised if I was right or wrong about that guess though.
The doubling of available pressure with series pumps is easy to grasp. The doubling of available flow with parallel pumps is easy to grasp. I can see why you'd be skeptical of an almost doubling of flow potential with series pumps though. What can I say? Somehow I have to reconcile what I've seen on the BMW with the theory...