It is not "my mod", I posted the mod in #3332 for Tiago.
@tiago please join the discussion and share your impression of this mod.
The RC that I was referring to is the Time Constant to replenish the voltage due to leakage.
See diagram.
Vcom ideally should stay constant. Practically it is performed by the (equivalent) resistor R and capacitor C.
I don't know why these values were picked.
I know nothing about op-amp frequency compensation.
I agree that the input impedance of both inverting and non-inverting inputs should not be varied too much.
But this is not possible due to frequency dependency of the impedance of Vcom capacitor C.
The 150uF wet tantalum cap mod also modified the input impedance by 15x too.
What is the spectrum of frequency of this Vcom capacitor C subjected to ??? Do you know ?
The output of the DAC is a sequence of sigma-delta spikes (I could be totally wrong here) which
may have all kinds of frequencies. This is my guess.