Initial (incorrect) calculations suggested water doesn't matter, its all about getting as much heat into the water as possible.
Once I found some mistakes in conversions, it seems water quantity and temperature does matter.
Essentially stemmed from "Why does thrasher/PRJ seem to hold a marginal benefit over ZZP?"
Namely bar/plate vs heater core. Seems a bar and plate would be ideal if it could be impossibly large, but we simply do not have the space.
Laminova cores come to mind, but those are precision pieces and quite pricey.
All leads back to my original assumption: Buy a neil core, cheap front mount, bosch pump (possibly meziere), and large reservoir in trunk. Tank being optional as well.