However, as Jose pointed out, RPM and/or MP will not remain constant as
you adjust the mixture. Just because the controls have not moved, that
does not mean that the power setting hasn't changed.

It's one of those "I just can't remember, exactly" things and I'm very much
grounded by my wallet at the moment, dammit. Although this could be a really
good excuse to get in and fool around "in the name of science". Better
justification than a $200 Coca Cola.

RPM with a constant speed should stay the same (or at least quickly revert
back to the same) shouldn't it? If so, then it's the MP that would vary as
you pull the mixture back. Off hand, do you remember by how much it varies
in a typical type you fly when you go from best power to best econ?