Can I improve flow rate by reducing diameter of a downpipe?

[jeff Simmons]

Hi, I’m installing a down pipe which goes from the house to a waste water treatment tank. The length of the route is 55 meters and the pipe diameter is 125mm (which collects black water from 5 toilets, grey waters are on a separate pipe). For about 35 meters the pipe has a good inclination of about 20-30° And the water flow seems to run well, but the last 20 meter section it only has around 2-3°, would it help if I reduce the last section to 110mm?
Thanks
Jeff

 

[exedon2]

Exactly the opposite larger the pipe less fall it needs.