Vivek,

  Should be no problem simulating the op-amp and
getting the noise floor you are targeting. However,
simulating the op amp by itself really is not meaningful
and you will probably need to simulate the entire
Delta-Sigma ADC to make the measurements you
need. It should be possible to look at the distortion
without running 2^20 points. Somewhere between
2048 and 16384 should be enough, look at the FFT
plots in the following document, they use 65536 point
FFT and have plenty of dynamic range.

http://www.imse.cnm.es/esd-msd/WORKSHOPS/MIXMODEST/PRESENTATIONS/medeiro2.pdf

Some other things to watch are:
1) You need to use windowing function when performing
    the  FFT, for example, the hanning window. Unlike
    Pipeline ADC, windowing is required when performing
    an FFT on a Delta-Sigma ADC
2) You also  need to control the simulation accuracy, try
    using Spectre's relref=pointlocal and
    method=gear2only

BTW, if you do this simulation at transistor level it will
take a while, filter the outputs to prevent filling up the
disk. Also consider strobing the outputs, again to minimize
the data size.


                                                         Best Regards,

                                                              Sheldon