That’s a good point. A better comparison should probably examine that baseline directly (the differences in batches of the same film).
So far, I’d say the evidence I have points to the differences between ProImage and Kodacolor 100 being greater than batch variation, but it’s not 100% conclusive. I’m referring to my earlier test: Kodak ColorPlus vs. Kodacolor 200 vs. Kodak Gold (analog.cafe/r/kodak-colorp…). I’m assuming that since ColorPlus and Kodacolor 200 use the same formula (as the above article explains). They are also quite likely to have been produced in separate batches (due to different production dates). And yet, both films render images that appear identical in my scans, whereas the third film in the mix — Kodak Gold — looks identifiably different.
All of that, of course, comes with a huge caveat: age, dev chemicals and methods, and, even more so, the scanning process, can erase or exaggerate the slight differences I found.
Dmitri