I see he's edited it to remove some of the most glaring errors but there is no such thing as "true impedance" and no "figure of merit" applicable to a passive and nor is there any way that specific values will affect the dynamic range... not without a compander hidden in the potentiometer!
The worst (highest) output impedance is at the electrical midpoint of the pot (assuming a low source resistance), not the opposite as LDR's post originally hinted.
To be pedantic, we are talking resistance not impedance when discussing a passive pot (and in this context specifically a passive pot), A few MHz would be the lowest frequency where impedance would be more suitable a term.
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