I have been runnng with my “ALPY 200” low resolution faint object spectrograph configuration again over the past few weeks and have been able to work my way down about a mag fainter to around 17.5.
http://www.threehillsobservatory.co.uk/astro/spectra_46.htm
I recorded the spectra of these two supernovae (asassn-15fa and asassn_15fz) a few weeks after discovery so they have evolved significantly compared the original spectra taken soon after they were discovered. The match with spectra of supernovae at a similar age is still clear though.
The supernova contribution is already much lower than the sky background which is significantly limiting the signal/noise. I am not sure if more aperture or darker skies will be needed to go much lower, probably a combination of both.
Robin