I have never seen a problem with water flow in a 6Gx waterpump.
The Mercedes auxiliary waterpump I an aware of is to provide flow to the heater core.

I do not see water flow being Edwards problem here, he simply does not have a radiator that cools his engine. The change he has in process will be noticeable.
The only other change I would consider is an active vent from the bleeder on top of the thermo housing to the low pressure side of the radiator. I install crossflow radiators so the vent is easy to incorporate. This way any gasses that form in the system are not trapped and blocking flow.