Is there only one fuel filter? There may also be water seperator? I would work on understanding what the issue is, before adding additional parts. An electric lift pump will not help if you have a blockage on the tank pick up pipe. You should not fit an electric lift pump without checking it...

Did you get this issue fixed?

It sounds like you may not be getting enough fuel to the high pressure pump - especially if you are underpowered at high engine speed. This would explain why moving the fuel limit screw is having no effect. Have you changed the the fuel filter(s), checked the fuel lines for restrictions...