When i ran an electric pump. i left the mechanical one in place. The fuel passed through both of them. I added the electric one when i had a small vapour lock problem one hot summers day in my 1000 sedan [with 1200GX] There was 4 of us in the sedan, towing a boat, which was loaded with all our camping gear & the outboard motor, as i was about to pull out to pass this slacker in a Commodore while coming up a steepish pinch on the Clyde mountain. The engine died for a second or so, then got with the programme. Those 5 speed option 1 boxes are magic.
The electric pump helped out with this problem & it never happened again.
Chris