This sounds like a project in one of the shops that produces custom cars from stock cars. The crews are great with engines and drive lines and suspension for handling, but not master engineers in electronics. But usually they have experience about what does not work!
And in an automotive installation there is seldom enough room for anything. And of course the whole engine compartment gets very hot.
And keep in mind that a pump load increases with the square of the speed increase.

 

This sounds very similar to a thread that was started a month or so ago, for a Mercedes if I remember right.

its a return-less “low pressure” pump that feeds the direct injection pump the module reads the fuel pressure and adjusts the pump speed for demand. It doesn’t have have a fuel pressure regulator. And I’ve confirmed it’s variable voltage
also it is not mapped it simply increases voltage until it sees proper fuel pressure.

Couldn't this bigger pump be controlled by the signal that is now controlling the original pump? Or adding a return line and regulator and just power the replacement pump with the ignition circuit?

 

If the current draw of the larger pump is known, and if the reducedvoltage that will be heating the pass transistors is known, then a set of power transistors on a heat sink can be the replacement driver, with the number of transistors selected to survive the dissipation. But they will need to be located elsewhere., in an area with good airflow. And one additional idea is to have a resistor in parallel with the control transistor to provide that lower pressure flow without the transistor carrying much load. Then the transistor could switch on when more power is needed. The extra shunt resistor would be what gets hot when the pump was running slowly. and power resistors are made to handle the heat. ( This really IS a cheating trick. It works well.)

 

The GM Pump Controller puts out a ~12 Volt, ~22-Amp, 20khz PWM Signal.

What is your expected Horsepower level ?,
and is that with Gasoline, or E-85 for Fuel.

What are the Current requirements for your proposed new Pump(s) ?
The Pump manufactures will usually supply a Graph
showing Current and Voltage vs Gallons per minute.
This could be very important information.

I decided to go with a "FueLab-Brushless-Pump" that accepts a 0 to 5-Volt input to control the speed,
but now I've swapped over to a MegaSquirt Computer because
I got tired of dealing with the Factory PCM, now there's fewer complications to deal with.

Do You already have a new Pump ?

What are the Specs ?

My advice would be to leave the GDI System just like the Factory delivered it,
and simply "Add-On" a Secondary Injector System,
supplied by a separate Fuel-Pump,
and controlled by a MicroSquirt Computer.
This arraignment is good for up to ~1500 horsepower with the right combination of other parts.

Otherwise, you'd wind-up building a dedicated "Race-Car",
or spending boat-loads of cash on a street-sleeper,
and then the strategy changes for every single part, including the Engine-Block.
.
.
.

 

[...] to have a resistor in parallel with the control transistor to provide that lower pressure flow without the transistor carrying much load. Then the transistor could switch on when more power is needed. The extra shunt resistor would be what gets hot when the pump was running slowly. and power resistors are made to handle the heat. ( This really IS a cheating trick. It works well.)

A trick I've used in the past - also switching in a bank of resistors with MOSFETs that have low Rds(on) - but you have to know the characteristics of the load fairly precisely to get the most benefit and I suspect that might be tricky (no pun intended!) here.