Effective load close to or less than cart impedance is what I think you mean is a problem...or by heavily loaded.
Yes. And I'm also saying that having a load impedance that is heavy enough (numerically low enough) to approach the impedance of the cartridge will almost invariably not be optimal sonically. And if a much lighter (numerically much higher) load impedance is applied, that is likely to be more optimal sonically, whether or not equation* is applied becomes insignificant.
... however the cartridge impedance is fixed and I was hoping to vary the step up in the analysis
Everything I said is consistent with that. Keep in mind that as the turns ratio of the step up transformer is varied two things change: The load impedance seen by the cartridge (which is equal to the input impedance of the phono stage divided by the square of the turns ratio), and the input voltage seen by the phono stage (which is equal to the voltage applied to the primary of the transformer, referred to as "Vout" in equation*, multiplied by the turns ratio).
Regards,
-- Al