I did measurements of ENOB for WM8731 ones, 15 bits approximately, with HPF "on". For DC I would expect no more than 11-12 bits. You may get better accuracy via implementing self calibration, adding precise DC reference and switch, than use uCPU to track DC drift in real time, interrupting DC voltage monitoring few times in a minute or so.
In overall, IMHO, it makes sense if 20-48 ksps is necessary.
It's not clear what is your application, first topic says 20 ksps than 5-10 sample per second was brought into discussion.