I would definitely expect someone with a faster metabolism to put out more total net heat, which is measurable with difficulty, and also consume oxygen faster (and produce carbon dioxide faster) which is measurable with some difficulty, but a lot less.

Yes, but that would require much more involved procedures than just taking peoples' temperature. So I will concede that my argument about variation in temperature, which is sort of a back-of-the-envelope way of getting at the problem, is weaker than I had thought.