Here are some flow numbers for the walbro’s, http://www.roadraceengineering.com/fuelpumpflowrates.htm
The Walbro “255” LPH will supply enough fuel for 4x950cc/min injectors that are maxed out at a fuel pressure of 58psi, which would be good for about 625 crank horsepower on a 4cyl. At 73 psi fuel pressure this pump is only good for 575 crank horsepower.
Where as the Walbro “255” HP will supply enough fuel for 4x1000cc/min injectors that are maxed out at a fuel pressure of 58psi, which would be good for about 675 crank horsepower on a 4 cyl. At 73 psi fuel pressure this pump is good for 700 crank horsepower.
These numbers are based on calculations not real world numbers.