What do you consider superinsulated? I'm a little north of you in Wisconsin, so have similar heating needs. My house has 4" foam under the crawl space and footings and outside the foundation, R30 walls, R90 ceiling, and R3 windows. It has 13,000 BTUH heat loss at 70 F inside and 20 F outside. I consider it well insulated, but not superinsulated.
Do the heat loss calculation, then calculate what slab temperature you need to provide the heat, then calculate what combinations of water temperature, flow rate, and tubing length you need to get that slab temperature.
