I just looked at Reg Riemer's site .. he writes:
Note! I ran bench tests on the 5M-GE and 7M-GTE fuel pumps. Results are as follows, pumps make same pressure with stock wiring but do differ slightly in volume. The 5M-GE pump put through about 7 % less volume per minute.
According to this comparision chart between mk3, mk4 and walbro pump:
http://www.jblmk3.com/id82.htm (scroll down) -- the mk3 fuel pump delivers 26 GPH, so if the 5mge pump delivers 7% less that would be 24 GPH. (90 litres per hour at 40 psi for the 5mge pump).
Walbro delivers 51 GPH (192 litres) an hour ... damn ... thats alot !!
I guess theres no point of replacing the 5MGE fuel pump with a 7mgte.... heck, if im gonna spend hours of pulling the fuel tank out (and replace it, since I have a leak about 50% full), then it seems like a much better idea to install some kind of Walbro pump ...!