So, it's been long enough since the radiant floor heating was switched on (downstairs tenant using the furnace) that I have a picture of how it's affected my power use:
I compared 3 days this week against days in the pre-heated floor era where the daily low temperature was within +-1C of each of the recent 3 days.
That worked out to comparing 3 days to 21 previous days this winter.
The result: average of
16% lower energy usage since the radiant floor tenant moved in!
And that's without me turning down the thermostat. But with the superior comfort of a radiant floor, I think I can twist the dial back another notch or two without sacrificing any comfort. I didn't want to do this until I had some "controlled" stats, but now that I've got those, even more savings are in the works.