Sampling: how to determine a "good" sampling rate based on a given filter
There are other reasons for over-sampling. First is if you want to reconstruct the signal back into the analogue domain you have an easier job designing the DAC output filter. Remember that the process of digitization does "corrupt" the signals closer to half sample rate and that corruptions lowers the higher frequency amplitudes by a few dB - the reconstruction filter needs to be designed more carefully the closer you run to the half sample rate.

Another reason might be the ability to get "process gain" - this can significantly reduce the SNR incurred during the digitization process and improve overall SNR. Your question....


  What's a "good" sampling rate for a given filter of a given order e.g.
  the Butterworth of order 7?


... is unanswerable based on what I've said above.