Sooch,
Thanks for the feedback.
A higher flow lift pump, would not hurt anything, and would keep the tandem pump fully supplied with fuel, and this MAY (probably would?) increase recirc flow rates and keep the fuel cooler at higher RPM. The Pierburg pump that is installed seems to be a rotary positive displacement pump, not a solenoid style pump, such as the Walbro's appear to be (going by other's descriptions - I have never seen one.)
It would be important to keep all of the design features of the present fuel canister assembly, or accept the loss of some capabilities.
I would keep the pressure regulator that would hold the maximum pressure at about 15 psig to the suction of the tandem pump.
I would keep the eductor, and inlet check valve if at all possible. This actually does two functions; it keeps the canister full of fuel to help handle angles at very low fuel levels, and it pulls in excess cool fuel (or at least fuel at tank temperatures, rather than recirc line temperatures), allowing the warm return fuel to spill out the top of the canister, rather than going down to the pump suction.
Another possibility would be to install a bypass suction line, so that at really high demand, the tandem pump could pull fuel from the tank itself. I think this line would entail a third fuel line connection on top of the pump assembly, and a line going through the canister to pull from the tank at the bottom of the canister. This line would bypass the spring loaded check valve built into the top of the assembly, and would have to have a simple diaphragm check valve in it. This approach would mean that the PD owner would have to be as cautious of air inleakage as the ALH owner, or he could entrain air at high engine speeds and loads. This is the approach I was thinking of for myself. The most problematic area of this is liable to be the bulkhead penetration and sealing for the third line on top of the fuel pump plate.
A variation of the bypass line, would be to put an external (maybe Walbro or Carter solenoid style pump) that would, because of its design, only draw power when pressure dropped off in the filter supply line - normally at high RPM and high flow. This puts the two pumps in parallel, without having to put in another in-tank pump.
If I was racing, or really interested in high performance I would go with a system that maintained a positive pressure into the tandem pump, at all engine speeds at full fuel demand. This would mean higher electrical power consumption, and some loss in mileage - probably very minor. Heavier wiring may be required to the fuel pump. I do not know if the fuel pump relay can handle the increased current flow.
Remember that the flow through the orifices will vary with the square of the differential pressure across them. So for instance the minimum acceptable 51 psi tandem pump discharge pressure at 1500 rpm, and the 14.5 psi backpressure on the fuel rail give you a 36.5 psi difference across the orifices. The maximum pressure out of the tandem pump is 109 psi, giving you a maximum differential pressure of 94.5 psi. This works out to a maximum flow that is only 1.6 times the available flow at 1500 rpm. So there are limits to the benefits of maximizing suction pressure to the tandem pump.
Heat load from increased combustion may simply increase faster than we can increase fuel flow for cooling.
I usually cruise at speeds (73mph indicated, 70 actual) and power levels that do not cause any elevated fuel temperatures, so this is all a labor of curiosity for me - I have no performance issue that needs correction. This means I am not pursuing this with my usual goal oriented single mindedness (more descriptive and less kind words are often used by observers), or even average human vigor.
Dan