You said, "input power subjects the experiment to a poor signal to noise ratio based on noise in the output measurements."
I probably should have said that low output power is more subject to errors in measurement than higher power outputs. You will say, sometimes high power is hard to measure. I am saying that output power in the milliwatt range is much more difficult to measure accurately in real world situations by calorimetry, than is power in the dozens or hundreds of Watts.
Input power matters because it translates to output power. Although (usually) input power can be measured very accurately, that is not so true of output power. Small fractional errors in output power measurement become problematic when input power >> signal.
That is exactly what I was addressing. Thanks for making it clear.
"What you are missing is that small increases in output power compared to blanks indeed can subject the experiment to a poor signal to noise ratio based on noise in the output measurement. If the out/in ratio is very poor, then the input power contributes to the output noise because more power is needed to run the experiment."
I probably should have simplified this to something like: low level measurements of heat by calorimetry at some point become more error prone than higher level measurements. By noise, I didn't mean noise in the sense the word is used in data transmission or communication. I meant and should have said errors instead of noise. I never said input power is noise nor does your quote say that.