Good thing I like talking to myself
Ran some more calculations using this nifty link
Relative Horsepower Calculator
Based on that, it appears we would expect uncorrected values to be only about 1.3% lower for the higher humidity run -- but they are much lower than that (around 3.5% lower).
Assuming my rationale and subsequent maths are right, this further supports my tentative conclusion that the untuned catted Z simply makes more power with the Nismo take off than a tuned catless Z with the S-tune exhaust when conditions are equal (meaning, the difference between them cannot be accounted for based on ambient conditions alone).
Moral of the story and recommendation when searching for a bang-for-the-buck exhaust: Pass on the Nismo S-tune, buy the Nismo take-off.
Additional implication of these data: H-pipes > Y-pipes.