Wow Danny, that is really cool. There is some interesting pump tech out there and I may have to give it another look.
from a fluid mechanics point of view, at the downstream side of the orifice, the flow is in an unstable condition, with pressure increasing in the direction of flow. You need to be "gentile" to the fluid in this condition to prevent loss inducing secondary flows and separation from the surface... hence the long lead out. Upstream of the orifice, you are accelerating the fluid, meaning that the pressure gradient is negative in the direction of flow, which is a stable flow regime to be in, so you can be more aggressive with the changes in geometry...
