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I don't know either! I guess some do and some don't.
Some further thought. (I am not an evening person and can be a bit slow!).
There are two potential issues here with effective aperture:
a) Does the lens adjust the readout for aperture by allowing for its extension? Probably.
b) (Which I overlooked above). The 25 diopters will affect the magnification and, thus, the effective aperture. So many times, I have seen it stated that a supplementary lens does not affect exposure but this is untrue, any given change in magnification (by whatever means*) having an identical effect on exposure and any potential diffraction from the reduced aperture (real or effective, including any crop factor).
So, my figure for effective aperture, as arising from use of the 2xTC was correct but it was not the whole story and your calculated value looks about right, except that you should use 7 instead of 3.5, to allow for the crop factor.
When it comes to practicalities, I am interested in the FOV and DOF, plus diffraction, when it arises.
Thanks for the nudge.