well, I suppose that I do not have an answer of that scenario, because I cannot imagine it occurring. in a standard GA aircraft, you are not going to fly far enough, or for long enough to encounter enough atmospheric change to cause such an engine performance change. indeed, I don't think that a 172 could operate in conditions varied enough to produce such a performance change!
I doubt that in the real world of GA flying, you're going to encounter an operating change enough to affect engine operation at one power setting detectably. Probably the change in weight resulting from fuel burn will be noticeable first.
As I have previously suggested, unless you're flying a perfectly set up fuel injected engine, you're never going to optimize fuel flow/mixture to the extent you propose. Bear in mind that the C 172 S Flight Manual instructs that "lean" is 50F cool of peak, and operation lean of peak is prohibited. This is not the first engine for which I have seen this prohibition.
As I have said, understanding the concepts is good, understanding the limitations, and working within them is vital and required. Saving that last drop of fuel is not worth the effort, or the risk of operating the engine outside it's approved parameters. I would expect that a good wash and wax will have more noticeable affect on the cruise performance and economy of the flight, than attempting to optimize the mixture to the "enth" degree....