My chiller ended up in another country (resolution is imminent though) and so I am left playing with W estimates from water temps. I replumbed the loop so that temperature probes are closer to wb inlet and outlet, insulated the probes, and insulated the hoses. The result was a W estimate that is substantially higher than before, but that STILL isn't constant over a very large range of flow rates:
I am using a Cascade now instead of MCW5000A, and CPU temperatures were quite a bit lower than in the first run. I shall try to get another run in tonight; consider this very preliminary.