I wouldn't be worrying about heating capacity. From the spreadsheet, it looks like the heat pump can keep up with the load down to -8 degrees. As you said, the ground is 5 degrees warmer than the software model. Plus you're burying extra pipe for the loop. If it's -8 out, mama gonna shiver just looking out the window. I'm in Kentucky, and I know it hasn't been that cold here in a few years. Didn't hit Negative digits last winter at all.
You know what to do if you want warm floors. It involves wasting a little supply air or laying more pipe to put heat there. As with everything insulation, more is better if you do it correctly.
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