No, there are no issues under load or highway speeds because the pump has no problem supplying enough pressure to open the pressure regulator and operate the syphon jets. If it couldn't then there would be no syphon jet operation. It's that the pressure ends up being lower on an empty tank because there is not enough volume bypassing the pressure regulator to open the syphon jet relief valve which normally makes the back-pressure to the fuel pressure regulator constant. A full tank increases the back pressure on the syphon jets (back side of the rail pressure regulator) and increases the rail pressure. Would be easier to understand if we had a nice diagram.

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)