Common mode filters effectiveness
In the case of three series impedance's what will make the most difference to the total impedance? The largest impedance.

So if we have three series impedance's 

\$ Z_{total}=Z_{source}+Z_{filt}+Z_{load}\$

if the source and the load are significantly smaller than filt, then the filter will be biggest contributor to current

In the parallel case we get this:

\$ Z_{total}=Z_{source}+\frac{Z_{filt}Z_{load}}{Z_{filt}+Z_{load}}\$

and using a capacitor for the filter is sufficient to change the overall filtering.

IF either of those don't work, then a multi-element filter will need to be used, and the calculating the impedance will be complex.