Maybe it would be a good idea to recommend a small hole (3/32" or so) be drilled through the plug, though I'm still a little skeptical of the theory.  Did you attempt to slow the water pump rotation with different pulleys?  Maybe that would account for the lesser flow.

As a side note, has anyone else noticed that it's getting hard to remember the details of all the projects going on?  This is probably good news, that there's more projects, but it could just be advancing age :-)

Here's hoping for a huge air pocket.

Rusty