So let's do the math:
1) System Voltage not to exceed 13.5V (because a quick Google search yielded no flow data for higher voltages)
2) Lowest fuel rail pressure we may reasonably set is 40psi. (lowest rail pressure is worst case- flow rate drops at higher rail pressures)
3) In idle (worst case) essentially all fuel going into the rail also runs through the return line.
4) Length of return line is 7' (because Miata and I could only find 7' and 14')
5) ID of our 6mm evap line is 4mm
6) ID of 6an hose is 8.6mm
In this case Pats pumps would push approx. 665lph/176gph.
Walbro 400 (400lph @ 40psi):
Walbro 255 (265lph @ 40psi):
Now let's take that and plug it into the back-pressure curve for a 6an fuel line:
That yields approximately 5.5psi pressure drop in the return line. Which apparently Pat's regulator can handle. Here we have an actual datapoint- we can completely tune out 5.5psi back-pressure, using Pat's regulator- whichever that may be.
So if we're running just a Walbro 255 at 70gph, A random internet calculator
yields a pressure drop of 3.15 bar in a 7' long 4mm ID line. That's 46 psi. More than the previously assumed rail pressure- no good.
The same calculator with the same basic values, but an ID of 8.6mm for 6an hose gives us a pressure drop of 5.1psi which is pretty darn close to the 5.5psi pressure drop from the random internet graph, confirming that the assumptions for our back-pressure calculation agree with somebody else's reality.
So the full flow rate from a Walbro 255 seems too much for our NB evap lines. Again, making all the assumptions above and a few more.
Now somebody has a tank of gas, a Walbro 255, a pressure gauge and some evap line lying around to confirm?