...so much for sloppy methodology!

I redid the ISO tests between the 5 YO FF dSLR Sony a850 and the newish APS-C mirrorless Sony a6000. This time I carefully controlled the exposure to minimize normalizing the color checker chart after the fact.
The a850 was the control and required 0 EV compensation in ACR whereas the a6000 generally required -0.3 EV calibration to get the same grayscale when using the same exposure setting.

The takeaway from this is not only the equal noise levels but the nature of the noise is EXTREMELY similar. I'm choosing ISO 3200 because this is more similar to the original test at 1600.

I tend to use ISO 6400 only in abject desperation, so it isn't shown here.

All at 100% crop (no need for the 300% resampling in the last test)
Sharpening was turned off in ACR and USM (200, 0.4, 2) applied in post.